Add ruby-head and jruby-head to Travis, but allow them to fail

This way we'll know if/how the build is failing on the latest versions, but
without marking the entire build as a failure if only the head versions fail.

See:
  - https://github.com/rspec/rspec-core/blob/master/.travis.yml
  - http://blog.travis-ci.com/2013-11-27-fast-finishing-builds/