use github helper for dress_code gem in gemfile

Change-Id: I36af600183c33c47aac4547651db49362c52a91d
Reviewed-on: https://gerrit.instructure.com/40183
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Colleen Palmer <colleen@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>