fix user_json last_login for rails 3

Change-Id: Ibbeba429a69f9cf066ecb253a1456cb2d832aca7
Reviewed-on: https://gerrit.instructure.com/30679
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>