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.