fix conversation delete_for_all for rails 3

Change-Id: I81540efad41bd6e8d5e128106797980090d334f5
Reviewed-on: https://gerrit.instructure.com/30879
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>