pin node-sass at 0.9.3

the newer version we were using (0.9.5-rc1) seems to have a bug
that causes it to drop 'left' and 'right' styles

test plan:
 - the 'HTML Editor' and 'Rich Content Editor' links on the wiki
   page editor should appear on the right side and should not
   overlap the page title

fixes CNVS-15760

Change-Id: Icab9be6b25114485f0f5d9990b1eb3c1d600b35f
Reviewed-on: https://gerrit.instructure.com/41720
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ryan Shaw <ryan@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>
QA-Review: Ryan Shaw <ryan@instructure.com>