Running Bourbon with sassc

Hi,
This is a quick patch for running Bourbon with [sassc](https://github.com/hcatlin/sassc). I'm using `sassc` and Bourbon with this patch and the speed improvement is amazing compared to the classic `sass` executable.

The problem comes from `@-webkit-keyframes` and string concatenation in `url()`.
This is clearly a [libsass](https://github.com/hcatlin/libsass) problem on the [document_parser](https://github.com/hcatlin/libsass/blob/master/document_parser.cpp) but this patch can bypass this problem without breaking anything in actual code or performance issues.