[RFC] Update luarocks: Use commit with correct version number.

Luarocks 2.2.0 release has a wrong version number, which is fixed in later commits (https://github.com/keplerproject/luarocks/commit/ffba28653288f695515910f156b8dd4d74cc4005, https://github.com/keplerproject/luarocks/commit/0587afbb5fe8ceb2f2eea16f486bd6183bf02f29). There are are [no other commits in the history since the 2.2.0 release](https://github.com/keplerproject/luarocks/compare/v2.2.0...master), so it is safe to use them.