Replace usage of default.LIGHT_MAX with minetest.LIGHT_MAX

It was moved a long time ago and the former is not guaranteed to be available.
fixes #424

mesecons/oldwires.lua

@@ -30,7 +30,7 @@ minetest.register_node("mesecons:mesecon_on", {
 	},
 	groups = {dig_immediate=3, not_in_creaive_inventory=1, mesecon=1},
 	drop = "mesecons:mesecon_off 1",
-	light_source = default.LIGHT_MAX-11,
+	light_source = minetest.LIGHT_MAX-11,
 	mesecons = {conductor={
 		state = mesecon.state.on,
 		offstate = "mesecons:mesecon_off"