Fix travis builds on latest rubies

As of bundler 1.14.0 installing rainbow gem fails due sto bug in
rubygems that was fixed. For details see rainbow issue discussion:
ku1ik/rainbow#48 (comment)

Author: ixti

.travis.yml

@@ -2,6 +2,7 @@ language: ruby
 sudo: false
 
 before_install:
+  - gem update --system
   - gem install bundler
 
 bundler_args: --without development doc