Enable installation of a custom JDK version

Heroku's Java applications have the ability to choose the JDK.  Given that jRuby runs better on Java 1.7, I wanted this to also be a part of the jRuby Buildpack.

This pull request copies the JDK installation step from Heroku's Java Buildpack. The`compile` file for which is here:
https://github.com/heroku/heroku-buildpack-java/blob/master/bin/compile

The steps to enable OpenJDK 1.7 are described here:
https://devcenter.heroku.com/articles/add-java-version-to-an-existing-maven-app