Cache Bundler on Travis

http://docs.travis-ci.com/user/workers/container-based-infrastructure/

With Container based CI, public projects can cache the results of bundle
install

.travis.yml

@@ -22,3 +22,4 @@ matrix:
     - rvm: rbx-2
     - rvm: rbx-2
 
 
 sudo: false
 sudo: false
+cache: bundler