Update Rubinius version to 2

* rbx-19mode is deprecated by Travis CI: http://docs.travis-ci.com/user/languages/ruby/#Rubinius
Silence deprecation warning from Virtus

Virtus emits a warning:

> including Virtus module is deprecated. Use 'include Virtus.module' instead
Bump Virtus version to 1.0
Merge pull request #23 from sgerrand/deprecate-rbx-19-mode

Update build matrix to use Rubinius v2
Merge pull request #24 from sgerrand/virtus-1.0

Bump Virtus dependency to 1.0
Set required Ruby version to 1.9.3 and above
Merge pull request #25 from sgerrand/remove-mri-ruby-1.9.2

Set required Ruby version to 1.9.3 and above
Add MRI Ruby v2.1.1 to build matrix
Add MRI Ruby at HEAD to build matrix