Remove useless instantiations.
Add support for play 2.1.4+ drop support for play 2.1.3--.

* Bump the play shim plugin to make Play 2.1.x act like Play 2.2.0
* Add Play 2.2.0 probe features into sbt 0.12 probe.
* Create new test to ensure hooks work.
* Bump dependency of play shim to 2.1.5.
Tweak comments.
Merge pull request #42 from sbt/wip-jsuereth-play-215-support

Support Play 2.1.4+.