Simplified last modified check solving double event emission on OSX

I couldn't figure out the reason why there are checks for `isDirectory` or `nlink` so I removed them, because in any case, when the current `mtime` is equal to the previous `mtime` the function should return because that means there are no modification to the file. Right?

This should fix issue #48. I ran the test for the tree, and didn't error so I guess this should not break anything.