replace association with association_object in models

having an "association" attribute causes problems in rails 3

Change-Id: I3249a25cd2a6162ff33f548a2e9f4f51a2940ece
Reviewed-on: https://gerrit.instructure.com/29058
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>