Update mysql dependency to ~> 2.9
Add RuboCop
Fix RuboCop offenses
Enforce code coverage minimums
With the release of Rails 4, Rails 3.0 and 3.1 are no longer supported

See: http://weblog.rubyonrails.org/2013/2/24/maintenance-policy-for-ruby-on-rails/
Replace `should_receive` with new `:allow` syntax