[RFC] Force rocks.moonscript.org server for luarocks.

This forces luarocks to use the rocks.moonscript.org server, which was already used for some `luarocks install` commands and, additionally, currently seems to be the recommended luarocks repo server (see keplerproject/luarocks#276). This should fix #1013.
How about using `https://github.com/keplerproject/luarocks/archive/v2.2.0beta1.tar.gz` instead?