gem: add rubocop

Rakefile

@@ -1,6 +1,8 @@
 require 'bundler/gem_tasks'
 require 'rspec/core/rake_task'
+require 'rubocop/rake_task'
 
 RSpec::Core::RakeTask.new(:spec)
+RuboCop::RakeTask.new
 
-task :default => :spec
+task :default => [:rubocop, :spec]