Fix ev_ref / ev_unref warnings on later node versions

Hi baudehlo,

This patch fixes the deprecation warnings when using fs-ext on more recent versions of node. This might be a little rough -- I've checked the test suite and a couple of simple apps I have here on node v0.8.12, and both appear to behave identically both before and after the module is patched.

Feel free to do whatever you want with this patch.