allow bundler 1.7.6

Change-Id: If19271a01a997891bddb6614637154eae027b002
Reviewed-on: https://gerrit.instructure.com/44415
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Rob Orton <rob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>