fix canvas_update script sass compilation

We compile sass using npm run compile-sass now, but the
script was still referencing the old (now deprecated) rake
task, which was causing updates to fail.

Change-Id: Idbedcf61ccccbdb0f1deb5907afb25d5d6650000
Reviewed-on: https://gerrit.instructure.com/40392
Reviewed-by: Nick Houle <nhoule@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Dave Donahue <ddonahue@instructure.com>
QA-Review: Dave Donahue <ddonahue@instructure.com>