Skip ee_compat_check when merging to a non master branch

It would be nice to skip ee_compat_check when merging to a non master branch.

An example is https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/17748

We are merging update-csslab-version into csslab (staging branch for a few different MRs for reviewing). csslab has an EE MR ee-csslab but because update-csslab-version does not have an ee-update-csslab-version, it fails the pipeline.

We currently force merge it through because that failure is meaningless in our current scenario but it would be fantastic to make this job a little smarter (if possible)

/cc @rymai