allow bundler 1.7.10

so we can have platform: :mri_22 in the Gemfile

Change-Id: I9be28f3de61d1c7905c6574c7f402f2c7b20d47b
Reviewed-on: https://gerrit.instructure.com/46381
Reviewed-by: Jon Jensen <jon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>