Fixed cache busting for Rails 4

Before:
```ruby
if subdirectory.nil?
  bust_cache_path = generated_images_path
else
  bust_cache_path = generated_images_path + "/" + subdirectory
end
```
`subdirectory` will never be `nil?` there.
It will be `.` in case `path` is `icons-sXXXXXXXXXX.png` and `bust_cache_path ` becomes `app/assets/images/.`


Moving the sprited images into a deeper subfolder works around the problem, `subdirectory` becomes `folder_name` instead of `.`.


Fixes #94, #162, #187