Fix GitHub client requests when rate limit is disabled

## What does this MR do?

GitHub Rate Limit API returns 404 when the rate limit is disabled. In this case we just want to return gracefully instead of spitting out an error.

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

No.

## Why was this MR needed?

GitHub importer fails when rate limit is disabled in GitHub Enterprise.

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/697

## Does this MR meet the acceptance criteria?

- [X] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [X] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5191