All Rubies can behave like a JDK builder

A 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

Motivated by travis-ci/travis-ci#686

**Note:** JDK switch is possible for Ruby language, because multiple JDK versions are available on the Ruby worker VM.