fix Message#define_content template helper for rails 3

Change-Id: Ib51c6521f918d216612df74cf7e156dd95a74f2a
Reviewed-on: https://gerrit.instructure.com/28427
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>