Use new rubinius and don't use jruby-head on travis

Travis and Rubinius no longer use the 
rbx-18mode/19mode syntax for .travis.yml. 

http://rubini.us/2013/12/03/testing-with-rbx-on-travis/

.travis.yml

@@ -3,6 +3,5 @@ rvm:
   - 2.0.0
   - 1.9.3
   - jruby-19mode
-  - rbx-19mode
+  - rbx-2.1.1
   - ruby-head
-  - jruby-head