Convert canvas_cassandra gem to rspec 2

fixes: CNVS-11927

test plan:
  * automated test suite will cover this change
  * no production code is affected

Change-Id: Id79663b845f717c0a7ff64d73145f9866cbf7828
Reviewed-on: https://gerrit.instructure.com/32512
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>
QA-Review: Simon Williams <simon@instructure.com>