Update sass dependency to version 3.1.18
Update sass dependency to version 3.1.18

jasnow/follow-all,jasnow/follow-all,codeforamerica/follow-all,BryanH/follow-all,codeforamerica/follow-all,BryanH/follow-all,jasnow/follow-all,codeforamerica/follow-all,BryanH/follow-all