Make change in pull request #2 to also work on libsass.

The not expression in libsass is somewhat broken, but there is a workaround
presented in https://github.com/sass/libsass/pull/349 which fixes this issue for
us. It makes me feel a bit dirty, but so does a ruby dependency.
Merge pull request #3 from jvah/master

Make change in pull request #2 to also work on libsass.