Updates libsass to latest version

Hello

I tried to update libsass, as I ran into a problem with the currently included version of libsass. Unfortunately I've hit a few obstacles on the way, since upstream seems to have changed a little bit. I tried to adjust the code to my best knowledge and guesses.

Hope this pull request is useful!

Best wishes
Marcel Greter

P.s. I also made a pull request upstream:
https://github.com/hcatlin/libsass/pull/181