Use latest Rubinius (#80)

There don’t seem to binary versions of rbx-2 available on Travis any
more, so switch to using `rbx` instead.  Per travis-ci/docs-travis-ci-com#740, this seems to be the correct thing to do.

However, the `rbx` build is still failing.  This seems like it might be a temporary build environment issue, so allow the rbx build to fail until it is resolved.

.travis.yml

@@ -5,10 +5,11 @@ rvm:
   - 2.0.0
   - 2.1.0
   - ruby-head
-  - rbx-2
+  - rbx
 matrix:
   allow_failures:
     - rvm: ruby-head
+    - rvm: rbx
 before_install:
   - gem update bundler
 script: bundle exec rspec spec