enable picking srgb as default

fixes CNVS-10403

test plan:
 - turn on srgb in course settings
 - in gb2, click the 'switch gradebooks' link
 - navigate to courses/:id/grades
   - it should load srgb
 - click the 'switch gradebooks' link
 - navigate to courses/:id/grades
   - it should load gb2

Change-Id: I8c41b9763be44c2c968d818ac4d8a909476361a7
Reviewed-on: https://gerrit.instructure.com/29841
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Liz Abinante <labinante@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Cameron Sutter <csutter@instructure.com>