Bump sassc from 2.1.0 to 2.4.0
Bump sassc from 2.1.0 to 2.4.0

It contains a fix which makes function helpers thread-safe. Without this
fix, our CI fails to compile assets with sprockets 4.x, so we need this
change in order to update sprockets.

consul/consul,consul/consul,consul/consul,consul/consul,consul/consul