Update system gem in travis to avoid issue with rainbow gem

See ku1ik/rainbow#48 for more details.

Author: Ivo Anjo

.travis.yml

@@ -19,4 +19,8 @@ matrix:
     - rvm: jruby-head
     - rvm: rbx-2
 
+before_install:
+  - gem update --system
+  - gem install bundler
+
 bundler_args: --without development