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"

mesecons_button/init.lua

@@ -73,7 +73,7 @@ minetest.register_node("mesecons_button:button_on", {
 	legacy_wallmounted = true,
 	walkable = false,
 	on_rotate = false,
-	light_source = default.LIGHT_MAX-7,
+	light_source = minetest.LIGHT_MAX-7,
 	sunlight_propagates = true,
 	selection_box = {
 		type = "fixed",

mesecons_powerplant/init.lua

@@ -10,7 +10,7 @@ minetest.register_node("mesecons_powerplant:power_plant", {
 	is_ground_content = false,
 	walkable = false,
 	groups = {dig_immediate=3, mesecon = 2},
-	light_source = default.LIGHT_MAX-9,
+	light_source = minetest.LIGHT_MAX-9,
     	description="Power Plant",
 	selection_box = {
 		type = "fixed",

mesecons_torch/init.lua

@@ -80,7 +80,7 @@ minetest.register_node("mesecons_torch:mesecon_torch_on", {
 	paramtype2 = "wallmounted",
 	selection_box = torch_selectionbox,
 	groups = {dig_immediate=3},
-	light_source = default.LIGHT_MAX-5,
+	light_source = minetest.LIGHT_MAX-5,
 	description="Mesecon Torch",
 	sounds = default.node_sound_defaults(),
 	mesecons = {receptor = {