Update Rubinius build versions for Travis

At some point, Travis CI removed the `rbx-18mode` and `rbx-19mode` configurations for Travis, causing those builds to fail in the twilio-ruby matrix. This PR replaces these with Rubinius versions 2.1.1 and 2.2.1.

It also implements a workaround for a bug in Rubinius which requires the inclusion of the `rubysl` gem. This should be fixed in future RBX versions.