don't use a rake_tasks bundler group

it's nonstandard, and messes with spring

Change-Id: I6e9d4a663d5bfb94f58028613f70176eec23a637
Reviewed-on: https://gerrit.instructure.com/37827
Reviewed-by: Cameron Sutter <csutter@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>