Speed up rubocop CI job

Speed up rubocop CI job (Stage: Check) as each pipeline of omnibus-gitlab takes 25-35 min as of 2018-03.

Case: Pipeline #18681616

Whole jobs: https://gitlab.com/gitlab-org/omnibus-gitlab/pipelines/18681616/builds

Stage	Duration /s
Trigger Package	0
Check	308
Prepare	48
Tests	1318
Post Test	69
(Total)	1959