Fix sprite_image relative path

I was having some issues about sprite paths when I had a pretty uncommon directory and path configuration for my images and sprites. My generated sprites are placed in a directory inside the images path, not the image path it-self, so when the relative path returned by the `sprite_image` method was build it went over a extra directory upon searching the original sprite image.

I'm not really a ruby guy, so I don't know if something went missing, thought it's working for me on quite many different configurations I've tried before patching.

Just in case, I'm placing my *config.rb* settings at the time I had the problems, so anyone can replicate the error too:

```
relative_assets       = true
sass_dir              = "source"
css_dir               = "../css"
images_dir            = "../images"
generated_images_dir  = "../images/sprites"
sprite_load_path      = "sprites"
```

If you change the `generated_images_dir` to "../images" you will see it starts working. Odd.

Hope I can help ;)