Merge pull request #1397 from bf4/rbx_is_unreliable_in_ci

Closes #1396.  RBX just causes unnecessary CI failures

Author: bf4

.travis.yml

@@ -36,4 +36,5 @@ matrix:
     env: JRUBY_OPTS='--server -Xcompile.invokedynamic=false -Xcli.debug=true --debug'
   allow_failures:
     - rvm: ruby-head
+    - rvm: rbx-2
   fast_finish: true