only use half of available cores for compiling sass

...in the rake canvas:compile-assets task.

rake canvas:compile_assets does a lot of things in
parallel.  the slowest critical path is all the js
stuff. so by using all the cores for sass compiling
we slowed down the time till total completion.

the difference running it locally for me:
1:22.87 total vs 1:26.99 total

Change-Id: Ie919af7976b63ebaddf282178449ddf0f9fe917c
Reviewed-on: https://gerrit.instructure.com/38686
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>
QA-Review: Ryan Shaw <ryan@instructure.com>