don't write a non-attribute onto models

refs CNVS-15830

this is deprecated in rails 3, and raises an error in rails 4

test plan:
 * dashboard and course streams should still work

Change-Id: I180ae447ac7f384659d91d6e8382e5a7d5703576
Reviewed-on: https://gerrit.instructure.com/41788
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>