stub on the version-appropriate association class
stub on the version-appropriate association class

refs CNVS-9166

rails2.3 calls it AssociationProxy, rails3 just calls it Association.

Change-Id: I28d52d0a50a7f8d3269a92c64676c171c11a9239
Reviewed-on: https://gerrit.instructure.com/26233
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
Product-Review: Jacob Fugal <f862f167b85d41b225785c70d70808bc7337c1fe@instructure.com>
QA-Review: Jacob Fugal <f862f167b85d41b225785c70d70808bc7337c1fe@instructure.com>

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