bump rake version

for the OCD in all of us (and those that like to avoid
`bundle exec`)

Change-Id: Id4850f22a40b5a52b9a4ae89bccc3e774e17c834
Reviewed-on: https://gerrit.instructure.com/36313
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>