fix npm installs by pinning karma-coffee-preproccessor

Change-Id: I26809102e68d1be5951a2eb446f2f4a0aeb40e08
Reviewed-on: https://gerrit.instructure.com/31707
Reviewed-by: Aaron Shafovaloff <ashafovaloff@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Stanley Stuart <stanley@instructure.com>
QA-Review: Stanley Stuart <stanley@instructure.com>