Allow Rubinius to fail

For reasons we can't currently fathom, Rubinius has a tendency to fail
the build. Rather than let this continue to be the case, until such
time as someone has time to figure out precisely why, unblock other PRs
by letting builds pass with MRI Rubies for now.

Author: rgarner

.travis.yml

@@ -12,6 +12,7 @@ matrix:
   allow_failures:
     - rvm: ruby-head
     - rvm: jruby
+    - rvm: rbx-2
 
 before_install:
   - gem update --system