Gemfile: load REXML gem for brew bump

[bevanjkay]

• 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?

---

Currently our autobump workflow in homebrew-cask (and in my personal tap) is failing due to the :sparkle livecheck strategy requiring the REXML gem.
This started occurring after the Ruby3 on CI update, as REXML is no longer an included gem.

[Bo98]

Thanks!

[MikeMcQuaid]

Makes sense thanks @bevanjkay!