spec: canvas rspec rake task for vendored_gems

Change-Id: I8aceb8e5aa2b95790eadca83725c8fd03bd28965
Reviewed-on: https://gerrit.instructure.com/30621
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>