doc: update luarocks install instructions.

* README.md (Installation): Update to reference moonscript rocks
repository, which has better uptime than the default luarocks
domain used by released LuaRocks.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
maint: upgrade slingshot to work even when luarocks.org is down.

* slingshot: Upgrade to latest upstream.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>