fix #1046

The luajit GC is not reliable. On the emulator after turning 200 pages the memory usage went up to 200 MB. With this patch it reached a plateau after about 100 page turns.

Note that the increased memory usage on the green line is 30 MB of tile cache which means there is no noticeable memory leak any more. 

![memory_leak](https://cloud.githubusercontent.com/assets/751535/4811990/e864f100-5ebd-11e4-92ef-ea27426787e7.png)