Support for Play 2.1-RC2

The lastest [Play version](https://groups.google.com/d/topic/play-framework/m6w0deY9LMM/discussion) uses sbt 0.12.2-RC2.

This commit adds support to any sbt release candidate.

But there is still a problem because http://s3.amazonaws.com/heroku-jvm-langpack-scala/sbt-0.12.2-RC2.boot.properties is not found by [the compile script](https://github.com/heroku/heroku-buildpack-scala/blob/master/bin/compile#L121).