All Rubies can behave like a JDK builder

Non-JRuby project may rely on a specific JDK in their build chain.
Examples:
- https://github.com/ruboto/ruboto/blob/master/.travis.yml
- https://github.com/cassandra-rb/cassandra/blob/master/.travis.yml

Fix travis-ci/travis-ci#686
Merge pull request #158 from gildegoma/jdk_rubies

All Rubies can behave like a JDK builder