fix conversations controller for rails 3

Change-Id: Id21daa221f1ed984c7171e998b651322011c434f
Reviewed-on: https://gerrit.instructure.com/29872
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>