In-memory SQLite forgets DDL transaction

I've got some [code](https://gist.github.com/wsanchez/d29c7843852981f8cd14f5d886a47c28) that creates a SQLite DB, populates a schema, then writes some data.

If the SQLite DB is a file, this seems to work as expected:

```console
2018-06-08T16:10:40-0700 [__main__.Index#info] Importing transmissions...
2018-06-08T16:10:40-0700 [__main__.Index#info] Initializing SQLite DB...
2018-06-08T16:10:40-0700 [stdout#info] 2018-06-08 16:10:40,028 INFO sqlalchemy.engine.base.Engine SELECT CAST('test plain returns' AS VARCHAR(60)) AS anon_1
2018-06-08T16:10:40-0700 [stdout#info] 2018-06-08 16:10:40,029 INFO sqlalchemy.engine.base.Engine ()
2018-06-08T16:10:40-0700 [stdout#info] 2018-06-08 16:10:40,029 INFO sqlalchemy.engine.base.Engine SELECT CAST('test unicode returns' AS VARCHAR(60)) AS anon_1
2018-06-08T16:10:40-0700 [stdout#info] 2018-06-08 16:10:40,029 INFO sqlalchemy.engine.base.Engine ()
2018-06-08T16:10:40-0700 [stdout#info] 2018-06-08 16:10:40,030 INFO sqlalchemy.engine.base.Engine PRAGMA table_info("TRANSMISSION")
2018-06-08T16:10:40-0700 [stdout#info] 2018-06-08 16:10:40,030 INFO sqlalchemy.engine.base.Engine ()
2018-06-08T16:10:40-0700 [stdout#info] 2018-06-08 16:10:40,031 INFO sqlalchemy.engine.base.Engine INSERT INTO "TRANSMISSION" ("STATION", "SYSTEM", "CHANNEL") VALUES (?, ?, ?)
2018-06-08T16:10:40-0700 [stdout#info] 2018-06-08 16:10:40,031 INFO sqlalchemy.engine.base.Engine (('Radio 1', 'A', 'Ops'), ('Radio 4', 'B', 'Talk'))
2018-06-08T16:10:40-0700 [stdout#info] 2018-06-08 16:10:40,032 INFO sqlalchemy.engine.base.Engine COMMIT
2018-06-08T16:10:40-0700 [-] Main loop terminated.
```

However, if the SQLite DB is an in-memory DB, it fails, complaining that the table being written to doesn't exist:

```console
2018-06-08T16:12:34-0700 [__main__.Index#info] Importing transmissions...
2018-06-08T16:12:34-0700 [__main__.Index#info] Initializing SQLite DB...
2018-06-08T16:12:34-0700 [stdout#info] 2018-06-08 16:12:34,912 INFO sqlalchemy.engine.base.Engine SELECT CAST('test plain returns' AS VARCHAR(60)) AS anon_1
2018-06-08T16:12:34-0700 [stdout#info] 2018-06-08 16:12:34,912 INFO sqlalchemy.engine.base.Engine ()
2018-06-08T16:12:34-0700 [stdout#info] 2018-06-08 16:12:34,913 INFO sqlalchemy.engine.base.Engine SELECT CAST('test unicode returns' AS VARCHAR(60)) AS anon_1
2018-06-08T16:12:34-0700 [stdout#info] 2018-06-08 16:12:34,913 INFO sqlalchemy.engine.base.Engine ()
2018-06-08T16:12:34-0700 [stdout#info] 2018-06-08 16:12:34,914 INFO sqlalchemy.engine.base.Engine PRAGMA table_info("TRANSMISSION")
2018-06-08T16:12:34-0700 [stdout#info] 2018-06-08 16:12:34,914 INFO sqlalchemy.engine.base.Engine ()
2018-06-08T16:12:34-0700 [stdout#info] 2018-06-08 16:12:34,914 INFO sqlalchemy.engine.base.Engine
2018-06-08T16:12:34-0700 [stdout#info] CREATE TABLE "TRANSMISSION" (
2018-06-08T16:12:34-0700 [stdout#info] "STATION" VARCHAR NOT NULL,
2018-06-08T16:12:34-0700 [stdout#info] "SYSTEM" VARCHAR NOT NULL,
2018-06-08T16:12:34-0700 [stdout#info] "CHANNEL" VARCHAR NOT NULL
2018-06-08T16:12:34-0700 [stdout#info] )
2018-06-08T16:12:34-0700 [stdout#info] 2018-06-08 16:12:34,915 INFO sqlalchemy.engine.base.Engine ()
2018-06-08T16:12:34-0700 [stdout#info] 2018-06-08 16:12:34,915 INFO sqlalchemy.engine.base.Engine COMMIT
2018-06-08T16:12:34-0700 [stdout#info] 2018-06-08 16:12:34,916 INFO sqlalchemy.engine.base.Engine INSERT INTO "TRANSMISSION" ("STATION", "SYSTEM", "CHANNEL") VALUES (?, ?, ?)
2018-06-08T16:12:34-0700 [stdout#info] 2018-06-08 16:12:34,916 INFO sqlalchemy.engine.base.Engine (('Radio 1', 'A', 'Ops'), ('Radio 4', 'B', 'Talk'))
2018-06-08T16:12:34-0700 [stdout#info] 2018-06-08 16:12:34,916 INFO sqlalchemy.engine.base.Engine ROLLBACK
2018-06-08T16:12:34-0700 [-] main function encountered error
Traceback (most recent call last):
File "/Volumes/data/Users/wsanchez/Documents/Developer/BurningMan/radio-mixer/.tox/coverage-py36/lib/python3.6/site-packages/twisted/internet/defer.py", line 500, in errback
self._startRunCallbacks(fail)
File "/Volumes/data/Users/wsanchez/Documents/Developer/BurningMan/radio-mixer/.tox/coverage-py36/lib/python3.6/site-packages/twisted/internet/defer.py", line 567, in _startRunCallbacks
self._runCallbacks()
File "/Volumes/data/Users/wsanchez/Documents/Developer/BurningMan/radio-mixer/.tox/coverage-py36/lib/python3.6/site-packages/twisted/internet/defer.py", line 653, in _runCallbacks
current.result = callback(current.result, *args, **kw)
File "/Volumes/data/Users/wsanchez/Documents/Developer/BurningMan/radio-mixer/.tox/coverage-py36/lib/python3.6/site-packages/twisted/internet/defer.py", line 1442, in gotResult
_inlineCallbacks(r, g, deferred)
--- ---
File "/Volumes/data/Users/wsanchez/Documents/Developer/BurningMan/radio-mixer/.tox/coverage-py36/lib/python3.6/site-packages/twisted/internet/defer.py", line 1384, in _inlineCallbacks
result = result.throwExceptionIntoGenerator(g)
File "/Volumes/data/Users/wsanchez/Documents/Developer/BurningMan/radio-mixer/.tox/coverage-py36/lib/python3.6/site-packages/twisted/python/failure.py", line 422, in throwExceptionIntoGenerator
return g.throw(self.type, self.value, self.tb)
File "/Volumes/data/Users/wsanchez/Documents/Developer/BurningMan/radio-mixer/.tox/coverage-py36/lib/python3.6/site-packages/mixer/alch_test.py", line 94, in addTransmissions
for transmission in transmissions
File "/Volumes/data/Users/wsanchez/Documents/Developer/BurningMan/radio-mixer/.tox/coverage-py36/lib/python3.6/site-packages/alchimia/engine.py", line 33, in container
result = work(*args, **kwargs)
File "/Volumes/data/Users/wsanchez/Documents/Developer/BurningMan/radio-mixer/.tox/coverage-py36/lib/python3.6/site-packages/sqlalchemy/engine/base.py", line 2075, in execute
return connection.execute(statement, *multiparams, **params)
File "/Volumes/data/Users/wsanchez/Documents/Developer/BurningMan/radio-mixer/.tox/coverage-py36/lib/python3.6/site-packages/sqlalchemy/engine/base.py", line 948, in execute
return meth(self, multiparams, params)
File "/Volumes/data/Users/wsanchez/Documents/Developer/BurningMan/radio-mixer/.tox/coverage-py36/lib/python3.6/site-packages/sqlalchemy/sql/elements.py", line 269, in _execute_on_connection
return connection._execute_clauseelement(self, multiparams, params)
File "/Volumes/data/Users/wsanchez/Documents/Developer/BurningMan/radio-mixer/.tox/coverage-py36/lib/python3.6/site-packages/sqlalchemy/engine/base.py", line 1060, in _execute_clauseelement
compiled_sql, distilled_params
File "/Volumes/data/Users/wsanchez/Documents/Developer/BurningMan/radio-mixer/.tox/coverage-py36/lib/python3.6/site-packages/sqlalchemy/engine/base.py", line 1200, in _execute_context
context)
File "/Volumes/data/Users/wsanchez/Documents/Developer/BurningMan/radio-mixer/.tox/coverage-py36/lib/python3.6/site-packages/sqlalchemy/engine/base.py", line 1413, in _handle_dbapi_exception
exc_info
File "/Volumes/data/Users/wsanchez/Documents/Developer/BurningMan/radio-mixer/.tox/coverage-py36/lib/python3.6/site-packages/sqlalchemy/util/compat.py", line 203, in raise_from_cause
reraise(type(exception), exception, tb=exc_tb, cause=cause)
File "/Volumes/data/Users/wsanchez/Documents/Developer/BurningMan/radio-mixer/.tox/coverage-py36/lib/python3.6/site-packages/sqlalchemy/util/compat.py", line 186, in reraise
raise value.with_traceback(tb)
File "/Volumes/data/Users/wsanchez/Documents/Developer/BurningMan/radio-mixer/.tox/coverage-py36/lib/python3.6/site-packages/sqlalchemy/engine/base.py", line 1170, in _execute_context
context)
File "/Volumes/data/Users/wsanchez/Documents/Developer/BurningMan/radio-mixer/.tox/coverage-py36/lib/python3.6/site-packages/sqlalchemy/engine/default.py", line 505, in do_executemany
cursor.executemany(statement, parameters)
sqlalchemy.exc.OperationalError: (sqlite3.OperationalError) no such table: TRANSMISSION [SQL: 'INSERT INTO "TRANSMISSION" ("STATION", "SYSTEM", "CHANNEL") VALUES (?, ?, ?)'] [parameters: (('Radio 1', 'A', 'Ops'), ('Radio 4', 'B', 'Talk'))] (Background on this error at: http://sqlalche.me/e/e3q8)

2018-06-08T16:12:34-0700 [-] Main loop terminated.
```

The difference between these two runs is whether [line 71 or line 72](https://gist.github.com/wsanchez/d29c7843852981f8cd14f5d886a47c28#file-alch_test-py-L71) is commented out.