Support sidekiq-cron in gitlab.yml

Needs to go in 8.3 after gitlab-org/gitlab-ee!78 and gitlab-org/gitlab-ce!2087 get merged. cc @marin

@rspeicher this is blocked because gitlab.yml has not been finalized yet. It needs to go into 8.3.0 though.

@rspeicher be advised that the 2 mentioned MR's are not merged yet. I can add this in fast in the morning of the release but it will be (yet another) tight squeeze.

mentioned in merge request !569 (merged)

Status changed to closed by merge request !569 (merged)