Workaround an RVM bug in Travis which prevents the use of jruby-19mode, jruby-20mode and jruby-21mode

After my [initial report](https://github.com/travis-ci/travis-ci/issues/1947),
Travis folks [isolated the bug](https://github.com/travis-ci/travis-ci/issues/1948)
and reported it as [an issue to RVM](https://github.com/wayneeseguin/rvm/issues/2608)

Finally, we use jruby (that RVM resolves to jruby-1.7.0 in 1.9 mode) and jruby-head (that RVM resolves to jruby master in 2.1 mode).
Merge pull request #136 from Locale/workaround-rvm-bug-in-travis

Workaround an RVM bug in Travis which prevents the use of jruby-19mode, jruby-20mode and jruby-21mode