Fix date parsing/validity check

@kraman arbeitet bereits daran.

Seit 28.6.2015.

The tests are passing on CI, but this is in the console:

```
Deprecation warning: moment construction falls back to js Date. This is discouraged and will be removed in upcoming major release. Please refer to https://github.com/moment/moment/issues/1407 for more info.
Error
at deprecate (/Users/ryan/work/loopback-connector-sqlite3/node_modules/moment/moment.js:738:42)
at /Users/ryan/work/loopback-connector-sqlite3/node_modules/moment/moment.js:826:50
at /Users/ryan/work/loopback-connector-sqlite3/node_modules/moment/moment.js:8:85
at Object. (/Users/ryan/work/loopback-connector-sqlite3/node_modules/moment/moment.js:11:2)
at Module._compile (module.js:428:26)
at Object.Module._extensions..js (module.js:446:10)
at Module.load (module.js:353:32)
at Function.Module._load (module.js:308:12)
at Module.require (module.js:363:17)
at require (module.js:382:17)
at Object. (/Users/ryan/work/loopback-connector-sqlite3/lib/sqlite3.js:10:14)
at Module._compile (module.js:428:26)
at Object.Module._extensions..js (module.js:446:10)
at Module.load (module.js:353:32)
at Function.Module._load (module.js:308:12)
at Module.require (module.js:363:17)
at require (module.js:382:17)
at Object. (/Users/ryan/work/loopback-connector-sqlite3/index.js:1:80)
at Module._compile (module.js:428:26)
at Object.Module._extensions..js (module.js:446:10)
at Module.load (module.js:353:32)
at Function.Module._load (module.js:308:12)
at Module.require (module.js:363:17)
at require (module.js:382:17)
at global.getDataSource.global.getSchema (/Users/ryan/work/loopback-connector-sqlite3/test/init.js:6:27)
at Context. (/Users/ryan/work/loopback-connector-sqlite3/test/sqlite.dbdefaults.test.js:10:10)
at callFn (/Users/ryan/work/loopback-connector-sqlite3/node_modules/mocha/lib/runnable.js:266:21)
at Hook.Runnable.run (/Users/ryan/work/loopback-connector-sqlite3/node_modules/mocha/lib/runnable.js:259:7)
at next (/Users/ryan/work/loopback-connector-sqlite3/node_modules/mocha/lib/runner.js:271:10)
at Immediate._onImmediate (/Users/ryan/work/loopback-connector-sqlite3/node_modules/mocha/lib/runner.js:292:5)
at processImmediate [as _immediateCallback] (timers.js:371:17)
[Error: Invalid date default: '5']
```

**edit:** replaced deprecation with output from a non-coverage run of `npm test`