run screenreader gradebook tests in jenkins

stops ignoring screenreader gradebook tests for jenkins runs

test plan
=========
- retrigger the build a few times to be sure that failures haven't crept
  back in

Change-Id: I604d4fa533b2f525181f224db90a1ce654584208
Reviewed-on: https://gerrit.instructure.com/28538
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>
QA-Review: Simon Williams <simon@instructure.com>