Bust @stats cache of @trail

cachebust_generated_images didn't properly bust the following:

Sprites within a subdirectory such as:
  Rails.root /app/assets/images/sprites/somespritefolder/spritepictures[1234].png

Then a stylesheet such as:
   @import "sprites/somespritefolder/*.png";

Would generate the new combined sprite:
  Rails.root /app/assets/images/sprites/somespritefolder-s1934274242.png

But the asset_url's @trail would not be properly busted, so the new image could A) not be found, B) even when found, the @stats would say the file was not a file.

So:
```
# Delete the entries (directories) which cache the files/dirs in a directory
options[:sprockets][:environment].send(:trail).instance_variable_get(:@entries).delete(bust_cache_path) 
 
# Delete the stats (file/dir info) which cache the what kind of file/dir each image is
options[:sprockets][:environment].send(:trail).instance_variable_get(:@stats).delete(bust_image_stat_path) 
```

FYI:

    compass-rails (1.1.7)
      compass (>= 0.12.2)
      sprockets (<= 2.11.0)

    compass (0.12.6.15e51cd)
      chunky_png (~> 1.2)
      fssm (>= 0.2.7)
      sass (~> 3.2.19)

    sprockets (2.8.2)
      hike (~> 1.2)
      multi_json (~> 1.0)
      rack (~> 1.0)
      tilt (~> 1.1, != 1.3.0)