cask/cmd/upgrade: output cask upgrades on new line

[miccal]

• 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 tests with your changes locally?

---

This changes the output of which casks are to be upgraded from a single line separated by , to each cask to be upgraded being printed on a new line; this is consistent with how formula upgrades are listed.

This should also (hopefully) help with things like this.

Thank you.

[MikeMcQuaid]

Nice work again 👏🏻

[miccal]

Thanks @MikeMcQuaid -- a simple change, but a useful one I hope.