Stop testing Rubinius given this gem no longer supports it :(

Unfortunately, to count the number of methods, I'm using Ripper [1], a
Ruby script parser. Currently, Rubinius does not implement Ripper [2] so
I should probably:

1. Either use this feature only when the user is on some Ruby other than
Rubinius. This would allow using RubyCritic on Rubinius, but it would be
lacking a feature.

2. Or even better, switch from Ripper to Parser [3] and support all
features in all Rubies.

[1]: http://ruby-doc.org/stdlib-2.1.2/libdoc/ripper/rdoc/Ripper.html

[2]: rubinius/rubinius#2377

[3]: whitequark/parser#21

.travis.yml

@@ -4,7 +4,6 @@ rvm:
   - 1.9.3
   - 2.0.0
   - 2.1.1
-  - rbx-2
   - jruby
 
 script: "bundle exec rake"