Fix Shorthand linter with !important overrides

Fix bug in `Shorthand` linter where `!important` priority overrides
would prevent lints from being reported.

Solved by modifying the regex to exclude the `!important` declaration.

Change-Id: I53fee75da8590d43a4138df1fe6afabaa1e17f7d
Reviewed-on: http://gerrit.causes.com/37220
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>