Enable rubocop GitHub Actions formatter (#249)

Author: ntkme

Rakefile

@@ -23,4 +23,7 @@ end
 
 RSpec::Core::RakeTask.new
 
-RuboCop::RakeTask.new
+RuboCop::RakeTask.new do |task|
+  task.formatters = ['progress']
+  task.formatters << 'github' if ENV.key?('GITHUB_ACTIONS')
+end