Deprecate hoes to follow changes in Minetest Game

Hoes crafted from "rare" materials were deprecated in
9c459e77ac.

Hoes are still available in existing worlds, but they cannot
be crafted anymore.

This closes #29.

.luacheckrc

@@ -25,6 +25,7 @@ stds.minetest = {
 }
 
 read_globals = {
+	"farming",
 	"intllib",
 	"mg",
 }