use proper method for getting unserialized_value of a serialized column

fixes rails 4, where the signature for unserialize is different

Change-Id: I52c5fd9b3d7aaac7a4ccea0d9f40adf674e8732a
Reviewed-on: https://gerrit.instructure.com/41651
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>