MCO-519 rake rubocop action should have a meaningful exit status

Here we make the `rake rubocop` action exit with a status code that indicates
if rubocop is satsfied with the standards of the code.  Prior to this commit it
was simply printing the failing rules.

Rakefile

@@ -113,7 +113,7 @@ task(:rubocop) do
   if RUBY_VERSION !~ /1.8/
     require 'rubocop'
     cli = RuboCop::CLI.new
-    cli.run(%w(-D -f s))
+    exit cli.run(%w(-D -f s))
   else
     puts "Rubocop is disabled in ruby 1.8"
   end