Fix ferns compatibility with minetest >= 0.4.16 (#78)

ferns/crafting.lua

@@ -6,12 +6,17 @@ local S = plantlife_i18n.gettext
 -- (by Mossmanikin)
 -- License (everything):	WTFPL
 -----------------------------------------------------------------------------------------------
+
+local fern1 = minetest.registered_items["default:fern_1"] or false
+fern1 = (fern1 and fern1.name) or "ferns:fern_01"
+
+
 minetest.register_craft({
 	type = "shapeless",
 	output = "ferns:fiddlehead 3",
 	recipe = {"ferns:fern_01"},
 	replacements = {
-		{"ferns:fern_01", "ferns:ferntuber"}
+		{fern1, "ferns:ferntuber"}
 	},
 })