rails3 - update active_model_serializers to 0.9.0alpha

Updates ActiveModel::Serializers for rails 2 and rails 3.

Change-Id: I1203069cd921ca975788d5b3c2088bdcff3f5e77
Reviewed-on: https://gerrit.instructure.com/31113
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
QA-Review: Myller de Araujo <myller@instructure.com>
Product-Review: Stanley Stuart <stanley@instructure.com>