Remove JRuby and Rubinius from the travis matrix

This will save Travis some precious resource since it will not need to
run 18 jobs for these Ruby implementations on every push or pull
request.

We do care about these implementations but we should fix the build
locally before having it running on travis. We would love to have
someone working on this but right now it is not our reality.

.travis.yml

@@ -23,14 +23,10 @@ env:
 rvm:
   - 2.2.2
   - ruby-head
-  - rbx-2
-  - jruby-head
 matrix:
   allow_failures:
     - env: "GEM=ar:mysql"
     - rvm: ruby-head
-    - rvm: rbx-2
-    - rvm: jruby-head
   fast_finish: true
 notifications:
   email: false