Switch Rubocop hook to use extract_messages

Reduce the complexity of this hook by utilizing the `extract_messages`
helper now provided by the Hook::PreCommit::Base class.

This simplification will be applied to other hooks (like ScssLint) in
upcoming commits.

Change-Id: I761a3f0a1126bdce3ea7814a23b836ba2b2de8be
Reviewed-on: http://gerrit.causes.com/44900
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>
Switch ScssLint hook to use extract_messages

Remove some duplicate code found in other hooks by switching this hook
to use the `extract_messages` helper.

Change-Id: Ib0e1fb1b245cafd952ba34af8656ae9f0f2109df
Reviewed-on: http://gerrit.causes.com/44901
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>