simple buildbot_1 fails with ImportError: No module named psycopg2

When running `docker-compose up` with the simple example, I get the following over and over:

```
buildbot_1 | checking for running master
buildbot_1 | checking master.cfg
buildbot_1 | upgrading basedir
buildbot_1 | upgrading database (postgresql+psycopg2://buildbot:xxxx@db/buildbot)
buildbot_1 | Can't upgrade master yet. Waiting for database ready?
buildbot_1 | checking basedir
buildbot_1 | /usr/lib/python2.7/site-packages/buildbot/config.py:102: ConfigWarning: [0.9.0 and later] `buildbotNetUsageData` is not configured and defaults to
basic.
buildbot_1 | This parameter helps the buildbot development team to understand the installation base.
buildbot_1 | No personal information is collected.
buildbot_1 | Only installation software version info and plugin usage is sent.
buildbot_1 | You can `opt-out` by setting this variable to None.
buildbot_1 | Or `opt-in` for more information by setting it to "full".
buildbot_1 |
buildbot_1 | category=ConfigWarning,
buildbot_1 | /usr/lib/python2.7/site-packages/buildbot/config.py:102: ConfigWarning: [0.9.0 and later] NOTE: `status` targets are deprecated and ignored They are replaced by reporters
buildbot_1 | category=ConfigWarning,
buildbot_1 | problem while upgrading!:
buildbot_1 | Traceback (most recent call last):
buildbot_1 | File "/usr/lib/python2.7/site-packages/buildbot/scripts/upgrade_master.py", line 121, in _upgradeMaster
buildbot_1 | yield upgradeDatabase(config, master_cfg)
buildbot_1 | ImportError: No module named psycopg2
```
I tried installing `psycopg2` manually, but it fails because it can't find the `pg_config` executable.

```
/var/lib/buildbot # pip install psycopg2
Collecting psycopg2
Downloading https://files.pythonhosted.org/packages/74/83/51580322ed0e82cba7ad8e0af590b8fb2cf11bd5aaa1ed872661bd36f462/psycopg2-2.7.4.tar.gz (425kB)
100% |████████████████████████████████| 430kB 4.4MB/s
Complete output from command python setup.py egg_info:
running egg_info
creating pip-egg-info/psycopg2.egg-info
writing pip-egg-info/psycopg2.egg-info/PKG-INFO
writing top-level names to pip-egg-info/psycopg2.egg-info/top_level.txt
writing dependency_links to pip-egg-info/psycopg2.egg-info/dependency_links.txt
writing manifest file 'pip-egg-info/psycopg2.egg-info/SOURCES.txt'

Error: pg_config executable not found.

pg_config is required to build psycopg2 from source. Please add the directory
containing pg_config to the $PATH or specify the full executable path with the
option:

python setup.py build_ext --pg-config /path/to/pg_config build ...

or with the pg_config option in 'setup.cfg'.

If you prefer to avoid building psycopg2 from source, please install the PyPI
'psycopg2-binary' package instead.

For further information please check the 'doc/src/install.rst' file (also at
).

----------------------------------------
Command "python setup.py egg_info" failed with error code 1 in /tmp/pip-install-Q8qcHR/psycopg2/
```

When I try to install `psycopg2-binary`, it still tries to build from source (I'm guessing because none of the wheels are compatible?).

```
/var/lib/buildbot # pip install psycopg2-binary
Collecting psycopg2-binary
Downloading https://files.pythonhosted.org/packages/77/09/4991fcd9a8f4bea1ee3948e1729fa17c184d25bd10809bacc143626361b9/psycopg2-binary-2.7.4.tar.gz (426kB)
100% |████████████████████████████████| 430kB 6.4MB/s
Complete output from command python setup.py egg_info:
running egg_info
creating pip-egg-info/psycopg2_binary.egg-info
writing pip-egg-info/psycopg2_binary.egg-info/PKG-INFO
writing top-level names to pip-egg-info/psycopg2_binary.egg-info/top_level.txt
writing dependency_links to pip-egg-info/psycopg2_binary.egg-info/dependency_links.txt
writing manifest file 'pip-egg-info/psycopg2_binary.egg-info/SOURCES.txt'

Error: pg_config executable not found.

pg_config is required to build psycopg2 from source. Please add the directory
containing pg_config to the $PATH or specify the full executable path with the
option:

python setup.py build_ext --pg-config /path/to/pg_config build ...

or with the pg_config option in 'setup.cfg'.

If you prefer to avoid building psycopg2 from source, please install the PyPI
'psycopg2-binary' package instead.

For further information please check the 'doc/src/install.rst' file (also at
).

----------------------------------------
Command "python setup.py egg_info" failed with error code 1 in /tmp/pip-install-2kFAwj/psycopg2-binary/
```

I believe that `libpq-dev` needs to be installed: http://initd.org/psycopg/docs/install.html#build-prerequisites for this installation to work properly.