spec: make simply_versioned_spec more robust

Change-Id: I3730c467794b8bea998007eed14ed7d33fab9893
Reviewed-on: https://gerrit.instructure.com/41449
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>