Let Travis-CI automatically run luacheck on the game

.luacheckrc

+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

.travis.yml

+language: generic
+
+branches:
+  only:
+    - master
+
+sudo: required
+
+before_install:
+  - sudo apt-get update
+  - sudo apt-get install -y luarocks
+  - sudo luarocks install luacheck
+
+script: luacheck --no-color ./mods