Use stock LuaJIT

[AltraMayor]

It may be possible to review Gatekeeper's interface between C and LuaJIT to remove the need for luaL_pushcdata(), luaL_checkcdata(), and luaL_get_ctypeid(); these are the only functions required to integrate Gatekeeper and LuaJIT.

It seems that other projects have a similar need for those functions, so LuaJIT might eventually come up with a solution. If so, we'd be better off moving to the new solution.