bump rake gem

Change-Id: I751f667195264b4066ed852767c2240b4ef0db6f
Reviewed-on: https://gerrit.instructure.com/33642
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>