libsass: 3.4.8 -> 3.4.9

Semi-automatic update. These checks were performed:

- built on NixOS
- found 3.4.9 with grep in /nix/store/biismabsawadk87l7m83a62d8cj08iy1-libsass-3.4.9
- found 3.4.9 in filename of file in /nix/store/biismabsawadk87l7m83a62d8cj08iy1-libsass-3.4.9

cc "@codyopel @offline"