travis: drop rubinius

Because bundle install failed:

    Installing activesupport (4.0.0)
    Gem::InstallError: activesupport requires Ruby version >= 1.9.3.
    An error occurred while installing activesupport (4.0.0), and Bundler cannot
    continue.

    Make sure that `gem install activesupport -v '4.0.0'` succeeds before bundling.