attr_protected doesn't exist when you only include ActiveModel::Callbacks

Change-Id: I3903160ec24128fed732c3f8d4998d9842653bd8
Reviewed-on: https://gerrit.instructure.com/28705
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>