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.

appveyor.yml

@@ -21,6 +21,6 @@ install:
   - bundle install --retry=3
 
 test_script:
-  - bundle exec rake
+  - bundle exec rake test
 
 build: off