fix attachment clone_for for rails 3

Change-Id: I65e9030b8cff38ba2a45e7f5f49a934fa16e6342
Reviewed-on: https://gerrit.instructure.com/28690
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>