Remove rbx since it doesn't seem to install.

https://travis-ci.org/rails/rails-html-sanitizer/jobs/229025256


Don't have time to look into why, but if other's are reading this and want to add rbx back: please do investigate.

Author: kaspth

.travis.yml

@@ -10,7 +10,6 @@ rvm:
   - 2.3.3
   - 2.4.0
   - ruby-head
-  - rbx-2
   - jruby
 matrix:
   fast_finish: true