Add explicit YAML NULL to dummy en.yml

Rubinius's YAML parser can't handle the ambiguous value after `en:` in
the 'empty' dummy en.yml, so make the empty explicity by using the
canonical YAML NULL `~`.
Add rubinius to travis-ci build matrix

Add rbx-2.2 to fix the supported version of rubinius.
Merge pull request #14 from rapid7/feature/rubinius

Add Rubinius support