spec: rspec 2.13 parallelized_specs compatibility

Change-Id: I3600a84a6b7bde5bc6b25bfcd8ff448dfed4463f
Reviewed-on: https://gerrit.instructure.com/28736
Reviewed-by: Bryan Madsen <bryan@instructure.com>
Product-Review: Bryan Madsen <bryan@instructure.com>
QA-Review: Bryan Madsen <bryan@instructure.com>
Tested-by: Shawn Meredith <shawn@instructure.com>