META: Speed up CI pipeline

Currently each pipeline of omnibus-gitlab takes 25-35 min as of 2018-03.

(before) 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

(after) Case: Pipeline #19423627

Whole jobs: https://gitlab.com/gitlab-org/omnibus-gitlab/pipelines/19423627/builds

Stage	Duration /s
Trigger Package	0
Check	55
Prepare	53
Tests	1526
Post Test	68
(Total)	1702

TODOs

• Speed up rubocop job #3268 (closed) !2358 (merged)
• Speed up non-rubocop jobs without installing rubocop #3270 (closed) !2370 (merged)
• Skip to install google_drive gem in all jobs except for upgrade #3294 (closed) !2371 (closed)
• Speed up rubocop job with caching gems #3300 (closed) !2377 (merged)