Update: Aquamacs 3.0a.

Download URL on Aquamacs homepage is a redirect to GitHub. The
redirect URL was determined via `curl --write-out redirect_url`.
Caveats added to redirect users of 32-bit Intel-based Macs to
caskroom/versions/aquamacs25.
Merge pull request #2581 from goxberry/aquamacs-3.0a

Update: Aquamacs 3.0a.