fix avatar_helper for rails 3

Change-Id: I80c7559022cf25b747c068129e0ab8b804fe604c
Reviewed-on: https://gerrit.instructure.com/30962
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>