Add Play! framework buildpack - tested OK

Have tested using the standard Heroku Play sample: https://github.com/heroku/template-java-play

The inclusion of OpenJDK in the base stack has enabled this, as I suspected it would.

I expected the Scala buildpack to be working now as well, but encountered memory space issues - suspect more to do with the small VM I'm testing in more than any actual problem. May test the Scala buildpack later and submit a pull request, time permitting.