add rubocop binstub [ci skip]

bin/rubocop

+#!/usr/bin/env ruby
+#
+# This file was generated by Bundler.
+#
+# The application 'rubocop' is installed as part of a gem, and
+# this file is here to facilitate running it.
+#
+
+require 'pathname'
+ENV['BUNDLE_GEMFILE'] ||= File.expand_path("../../Gemfile",
+  Pathname.new(__FILE__).realpath)
+
+require 'rubygems'
+require 'bundler/setup'
+
+load Gem.bin_path('rubocop', 'rubocop')