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.

Rakefile

@@ -36,4 +36,4 @@ Rake::TestTask.new do |t|
   t.verbose = true
 end
 
-task :default => :test
+task default: [:test, :rubocop]