Display failing rule name

Otherwise how are we supposed to get more information about it?

Rakefile

@@ -5,6 +5,8 @@ require 'rspec/core/rake_task'
 RSpec::Core::RakeTask.new(:spec)
 
 require "rubocop/rake_task"
-RuboCop::RakeTask.new
+RuboCop::RakeTask.new do |t|
+  t.options = %w[--display-cop-names]
+end
 
 task :default => %w[spec rubocop]