Update Rubinius build setup

TL/DR; use rubinius 2.2.5 in development

Rubinius 2.2.5 now includes the standard library, so we do not need to
explicitly include standard library gems in Gemfiles.

http://www.benjaminfleischer.com/2013/12/05/testing-rubinius-on-travis-c
i/
Travis: remove Rubinius from allow_failures section
Merge pull request #753 from haml/rubinius

Update Travis setup for Rubinius