Require Lua v5.1 for now (since lua_getfenv() was deprecated in v5.2)

Seems a few APIs got deprecated, and our builds fail with Lua 5.2 and later (tested to fail with 5.2.2 at least). One suggestion would be, for now, to not build the Lua plugin if the version is >= 5.2. I have no idea how involved the real fixes would be.