fix user order_by_sortable_name for rails 3

Change-Id: I8d3ba6ddd3f6d9fd0a5dfba8bfab8a4419c8713c
Reviewed-on: https://gerrit.instructure.com/28628
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>