add support for postgresql 9.4
update project dependencies
update CI configuration:

* add Chef 12 support
* more bundler jobs
* cache bundler
* use docker infrastructure
* use latest available ruby 2.1
* drop ruby 1.9 support
* add `Gitter.im` notifications
update README flair

[ci skip]
Merge pull request #128 from phlipper/9.4

Add official support for version 9.4