Update Travis setup for Rubinius

This PR simplifies the Travis Rubinius setup and related Gemfiles and requires Rubinius to work. It updates the Travis version of Rubinius to 2.2.5+, which includes the ruby standard library. Thus we no longer need platform-specific Gemfile configuration for Rubinius.

In the process of doing this, I found a bug in Travis related to the `allow_failures` section (see https://github.com/travis-ci/travis-ci/issues/2004) that moved all the builds to the `allow_failures` section.

As the Rubinius build now works, I removed it from the `allow_failures` section, so the above issue is not a problem.

:computer: To run the tests locally, you will need to use Rubinius 2.2.5.

Please don't delete the `rbx-2` branch - I'm leaving it there for the Travis team.