Change default rake task to run test and rubocop

The rubocop only runs in the CI this way a contributor probably will see a
rubocop offense only in the CI.

Running the rubocop in the default rake task we have more chance that a offense
be get in the local machine.

.travis.yml

@@ -18,7 +18,6 @@ install:
 
 script:
   - env CAPTURE_STDERR=false bundle exec rake
-  - bundle exec rake rubocop
 
 env:
   - "RAILS_VERSION=4.0"