fix selective_content_formatter for rails 3

Change-Id: Ie26c2eb56d1e29c1108f13d9ef8d7773e908a298
Reviewed-on: https://gerrit.instructure.com/29606
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>