fix conversation add_message for rails 3

Change-Id: Ie07511d14005f26bc794e74e4f6a3d1f46544c47
Reviewed-on: https://gerrit.instructure.com/28343
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>