Delete stats for filename in case they were cached

This PR fixes issue  #158
Hike 1.2.3 not only caches the path entries, but also caches the file stats. This causes problem when the sprockets cache exists, and you recompile assets (because compass regenerates the sprite but hike already cached the app/assets/images path).

I've tested with:
* compass (0.12.6)
* compass-rails (2.0.0.pre d8ca1ef)
* hike (1.2.3)
* sass (3.2.19)
* sass-rails (4.0.3)
* sprockets (2.11.0)
* sprockets-rails (2.1.3)