fixes #7317 - skip rubocop rake tasks if library isn't installed

Package or package build environments won't have rubocop installed as it's in the test bundler group.  This skips load errors in the same way as jenkins.rake etc.

http://koji.katello.org/koji/taskinfo?taskID=147311 (:-1: nightlies without patch)
http://koji.katello.org/koji/taskinfo?taskID=147331 (:+1: with patch)