rspec 3-ify spec_helper

refs CNVS-16239

Change-Id: I60538bd7bc190ae32c527500d85ecdd5360f35f4
Reviewed-on: https://gerrit.instructure.com/42869
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Braden Anderson <braden@instructure.com>
Product-Review: Braden Anderson <braden@instructure.com>
QA-Review: Braden Anderson <braden@instructure.com>