fix messageable user and wiki_page for rails 3

after_find needs to actually be declared as a callback, not just a
method with that name

Change-Id: Ib61bebdc1379365fe3bccf5c3c4b3b9de2e347d7
Reviewed-on: https://gerrit.instructure.com/28743
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>