Style fix in cask audit

Co-authored-by: Markus Reiter <me@reitermark.us>

Library/Homebrew/cask/audit.rb

@@ -515,7 +515,7 @@ module Cask
     end
 
     def check_gitlab_repository
-      return unless @new_cask
+      return unless new_cask?
 
       user, repo = get_repo_data(%r{https?://gitlab\.com/([^/]+)/([^/]+)/?.*})
       return if user.nil?