Let Travis-CI automatically run luacheck on the game

.luacheckrc

@@ -0,0 +1,17 @@
+unused_args = false
+allow_defined_top = true
+
+read_globals = {
+	"DIR_DELIM",
+	"minetest", "core",
+	"dump",
+	"vector", "nodeupdate",
+	"VoxelManip", "VoxelArea",
+	"PseudoRandom", "ItemStack",
+}
+
+-- Overwrites minetest.handle_node_drops
+files["mods/creative/init.lua"].globals = { "minetest" }
+
+-- Don't report on legacy definitions of globals.
+files["mods/default/legacy.lua"].global = false