Remove rbx from CI. Allowing failures is just wasting resources

Breaks too often.

Happy to re-add if someone want to take charge of keeping it green.

Author: bf4

.travis.yml

@@ -8,7 +8,6 @@ rvm:
   - 2.2.3
   - 2.3.0
   - ruby-head
-  - rbx-2
   - jruby-9.0.4.0
   - jruby-head
 
@@ -49,5 +48,4 @@ matrix:
   allow_failures:
     - rvm: ruby-head
     - rvm: jruby-head
-    - rvm: rbx-2
   fast_finish: true