Gaek

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- Upgraded PyYAML version in test requirement for security fix.

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- Added `_safe` methods for functions which break outside of the GAE environment.

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- Added support for encoding `ndb.Key` objects as entities, pairs, or urlsafe-strings.
- Fixed a compatibility issue between `ndb_json` and `freezegun`.
- Fixed an issue where `'null'` could not be decoded properly.
- Fixed an issue where nested date strings were not decoded into `datetime` objects.

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- Fixed a bug in `environ.is_default_version()`
- Cleaned up `requirements.txt`

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- Build and file clean-ups.
- Configure Travis CI to auto-deploy to PyPi.

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- Patch version in order to replace uploaded sdist files on PyPi.