use an unreleased mocha on ruby 2.1

Change-Id: I3bb3a6e51fc5f76d5fc31a1c894e83ea5d5b22ec
Reviewed-on: https://gerrit.instructure.com/36737
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>