kill another version_number in cache key

refs CNVS-6763

Test plan:
  assignment due dates should display properly after updating an
  assignment's overridden due dates

Change-Id: I3f066444dc969b4ee0cd30fe823278852216ae1e
Reviewed-on: https://gerrit.instructure.com/28798
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>