Revert "Don't let rbx fail" - commit again when Travis has rbx 2.0.0rc1 or higher.

This reverts commit 6cf40a4.

Conflicts:
	.travis.yml

Author: sodabrew

.travis.yml

@@ -7,6 +7,10 @@ rvm:
   - rbx-18mode
   - rbx-19mode
   - ruby-head
+matrix:
+  allow_failures:
+    - rvm: rbx-18mode
+    - rvm: rbx-19mode
 bundler_args: --without benchmarks
 script:
   - bundle exec rake