spec: vendored_gems pin rspec versions

Change-Id: Ibad1c172e58a69eb3a5a9ff2c874e861086ae227
Reviewed-on: https://gerrit.instructure.com/35756
Reviewed-by: Bryan Madsen <bryan@instructure.com>
Product-Review: Bryan Madsen <bryan@instructure.com>
QA-Review: Bryan Madsen <bryan@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>