Fixes #53 SplFileInfo doesn't throw exception

The `_make_directory` function was failing due to an unused resource object.

Also, I couldn't find any evidence of PHP ever throwing an exception on `mkdir`, but I [left it in](https://github.com/kohana/cache/blob/3.3/master/classes/Kohana/Cache/File.php#L75-L89) just in-case.