Properly fix Rubinius dependencies in Gemfile & Travis integration.
Before Rubinius 2.2.0, some parts of the stdlib were lazily loaded, even if not included in the Gemfile. Now it's 100% required to include the gem 'rubysl', otherwise the stdlib is not available entirely, breaking everything. Also, the rubysl-test-unit gem was depending on minitest ~> 4.7, thus causing conflicts with Rails's dependency on minitest 5. That is not the case anymore since rubysl-test-unit 2.0.2, so it is completely safe to include it fully. When Travis is going to update RVM to 1.24, it'll be safe to use rbx-2, thus picking new versions automatically.