use ruby 2.1 compatible mocha with all versions of ruby

makes the lockfile the same between 1.9 and 2.1

Change-Id: I3ab1c3c7a7fcb8e6a5aba0170744d04739029209
Reviewed-on: https://gerrit.instructure.com/40753
Reviewed-by: Brian Palmer <brianp@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>