ci: Switch to GitHub Actions

After GitHub Actions were available, this repository kept using Travis
because it worked well enough, especially given this package is very
much in maintenance mode.  However, going forward it'd be nice to use
purcell/setup-emacs action to test various Emacs versions.

Don't bother testing against Emacs 24 because the next release will
bump the minimum Emacs version to at least 25.1.

While touching this setup, drop the Makefile target for downloading
dependencies and instead just expect dependencies to be one level up
from the snakemake-mode checkout.

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