Gemfile: Bump rubocop version for rake 11 compat
Gemfile: Bump rubocop version for rake 11 compat

Fixes `undefined method `last_comment'` errors running `rake`

LalatenduMohanty/landrush,njam/landrush,mheiges/landrush,vagrant-landrush/landrush,vagrant-landrush/landrush,phinze/landrush