Cache busting causes issues with rev and usemin

[tlaak]

The library is using Compass image-url($path, $only-path, $cache-buster) function without second and third argument, which means that it will always create a cache buster hash in the end of the image reference: background-image: url(my-image.png?1234567890);

This is a little bit problematic if you are using usemin and rev tasks with grunt to update the image references in css file to match the revved images, because the usemin will not match the image name if there is a hash included.