Luarocks: Cleanup

The formula is a mess. I’m not sure why the stable block was necessary
any more, upstream aren’t going to move on the install target check so
there’s no risk of the patch becoming obsolete any time soon.

This PR is pretty much just to make reading the Luarocks formula a
little more pleasant for everyone. Nothing more fancy than that for now.

It also turns out luarocks is using the system OpenSSL to check
download MD5Sums. I’m not sure whether this merits an additional
dependency or not. My gut says no, but equally I squirm at using the
system OpenSSL for anything. I’ll leave the OpenSSL point open to
discussion.
luarocks: update 2.2.0 bottle.