Update Libsass support

I don't know what the criteria is for documenting "libsass" support. Stable release? These seem to be fixed in the master branch:

- not operator: https://github.com/sass/libsass/commit/dc58506439edb940c2ad34248f186d29910497ca?diff=split
- random function: https://github.com/sass/libsass/commit/1bacd107ccdd155b90894f178db3d6d938d98ef5
- inspect function: https://github.com/sass/libsass/commit/4f905ad597d10aade0c5ae1d0584716f44d9cb86
- downward for: https://github.com/sass/libsass/commit/98a6ff3d3f3768be3da499e47f5db1c7a813212e