Use cache: policy: pull in .gitlab-ci.yml

What does this MR do?

Speeds up the CI run by skipping cache updates where not needed

Are there points in the code the reviewer needs to double check?

This can't be merged until GitLab.com is running 9.4 for both rails and runner

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?