Only install sass if it isn't installed already
Only install sass if it isn't installed already

This could all be handled more elegantly with bundler, but for the
moment this is a simple solution to speed up deploys.

Fixes #469

mysociety/yournextmp-popit,datamade/yournextmp-popit,YoQuieroSaber/yournextrepresentative,neavouli/yournextrepresentative,openstate/yournextrepresentative,DemocracyClub/yournextrepresentative,datamade/yournextmp-popit,openstate/yournextrepresentative,mysociety/yournextmp-popit,neavouli/yournextrepresentative,openstate/yournextrepresentative,datamade/yournextmp-popit,YoQuieroSaber/yournextrepresentative,DemocracyClub/yournextrepresentative,mysociety/yournextrepresentative,datamade/yournextmp-popit,YoQuieroSaber/yournextrepresentative,mysociety/yournextrepresentative,neavouli/yournextrepresentative,YoQuieroSaber/yournextrepresentative,neavouli/yournextrepresentative,datamade/yournextmp-popit,mysociety/yournextrepresentative,DemocracyClub/yournextrepresentative,mysociety/yournextrepresentative,mysociety/yournextmp-popit,neavouli/yournextrepresentative,mysociety/yournextmp-popit,openstate/yournextrepresentative,YoQuieroSaber/yournextrepresentative,openstate/yournextrepresentative,mysociety/yournextrepresentative,mysociety/yournextmp-popit