Update Ruby and JVM workers to new java cookbook

If cookbook update proposed in travis-ci/travis-cookbooks#236 is accepted, it is no more needed/possible to provision with `java::multi`, but it is enough to run `java::default` when the list of alternate versions is not empty.

@joshk: You'll still have to decide if you want to provision the same list of java versions on Ruby and JVM workers... Maybe it's too early to put OracleJDK8 on the Ruby box... (on the other hand you may prefer to avoid weird documentation, etc.). 

FYI: OracleJDK 8 use ~270M of disk space (like Oracle JDK 7)