moves Rubinus to allowed failures for Travis CI

  Rubinus was added to Travis recently, and has been
  failing intermittently since it was added. So for now,
  this commit moves it into the allowed failures until we can diagnose
  the problem.

.travis.yml

@@ -18,4 +18,5 @@ matrix:
     - rvm: jruby-18mode
     - rvm: jruby-head
     - rvm: ruby-head
+    - rvm: rbx-2
 sudo: false