rubocops/urls: allow plain HTTP mirrors

[Bo98]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

---

For the sources which we mandate HTTPS for, allow a plain HTTP mirror to be used, as long as the HTTPS version is present and is higher in the mirror list.

[BrewTestBot]

Review period skipped due to critical label.