fix plugin setting after_initialize for rails 3 compatibility

use a callback instead of overriding after_initialize

Change-Id: I7ea4a294ddf425b28d79572006e76aafc010dbc1
Reviewed-on: https://gerrit.instructure.com/28861
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>