[backport to `v0.16.x`] Change `source.url` in `rockspec` to use `git+https://` instead of `git://` protocol (deprecated by GitHub)

[milesrichardson]

This PR is a backport of #270 to the v0.16.x "branch" (there is only a tag AFAICT). I created a new branch from the v0.16.1 tag. I tried to make the GitHub PR target this tag, but apparently you can't target tags, and there is no corresponding branch that I could find to target. So at the moment this PR targets the master branch (meaning you probably don't want to merge it directly).

Quoting from #270:

GitHub deprecated usage of git:// (unencrypted Git protocol) on March 15th, 2022, so LuaRocks will fail to install any package including a source.url starting with git://. According to the LuaRocks documentation, the correct syntax for referencing https:// URLs is git+https://.