Fix subbing similar path names while generating sprite map

There is a problem when compass project have multiple directories with assets and this directories have similar names, e.g.:
```
/long/path/project/app/assets/blocks
/long/path/project/app/assets/blocks-mobile
```
Without this fix, for the second directory name `sprite.include?(path)` return `true` when `path` is `project/app/assets/blocks` and sprite generation will be fail.