allow bundler 1.7.3

Change-Id: Icb252a2937ed1a55897afc0bbb9121749745dbe5
Reviewed-on: https://gerrit.instructure.com/42092
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>