Remove hack to force a rebuild of node-sass.
Remove hack to force a rebuild of node-sass.

The line was originally put in to address failures in the Node 8
upgrade where the Node 6 version was getting cached, but caused other
problems in Travis where the process doesn't have the permissions to
create those directories. Hopefully no longer needed.

edx/course-discovery,edx/course-discovery,edx/course-discovery,edx/course-discovery