dependabot: allow indirect dependency updates

[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?
• Have you successfully run brew man locally and committed any changes?

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Old dependabot-preview used to do this. I think it's better for security reasons.

[BrewTestBot]

Review period will end on 2021-02-15 at 03:16:07 UTC.

[BrewTestBot]

Review period ended.

[Bo98]

This matches what the docs suggest to do, though the last docs check didn't seem to check for anything more: https://github.com/Homebrew/brew/network/updates/88921829

[Bo98]

Tried it on a test repo with the same Gemfile(.lock):

I guess something is bugged with this repo specifically 🤷‍♂️