Update compass

https://github.com/mysociety/alaveteli/pull/1942
I want to use the sprites in compass for compressing small icons in my theme. However when I use this code
``` 
@import "search/*.png";

.icon.human {
    width: 13px;
    height: 15px;
    @include search-sprite('human');
  }
```
in sass it breaks with an error because it searches the icons only in app/assets/images but not in lib/themes/alavateli-theme/assets/images .
This is some bug in the old compass-rails and for this reason I am making a pull request to update compass-rails.