Fix failing backup filename spec for RCs

See http://rubular.com/r/9oI7K8b773 for more details.

spec/tasks/gitlab/backup_rake_spec.rb

@@ -352,7 +352,7 @@ describe 'gitlab:app namespace rake task' do
     end
 
     it 'name has human readable time' do
-      expect(@backup_tar).to match(/\d+_\d{4}_\d{2}_\d{2}_\d+\.\d+\.\d+(-pre)?_gitlab_backup.tar$/)
+      expect(@backup_tar).to match(/\d+_\d{4}_\d{2}_\d{2}_\d+\.\d+\.\d+(-pre|-rc\d+)?_gitlab_backup.tar$/)
     end
   end
 end # gitlab:app namespace