Add .sass-cache to .gitignore

The Makefile removes this automatically, but compass watch won't do that
for you. Much better to just .gitignore it so we don't have to worry
about it.

See resources/mediawiki.ui/sourcefiles to see what I mean, and see
https://www.mediawiki.org/wiki/Agora/Engineering#Working_with_the_Compass_code
for documentation about the current approach.

Change-Id: Icf7dbc9e19f0615c5fcc88924a17ae695df0ff05
Merge "Add .sass-cache to .gitignore"