fix uri building in User.avatar_fallback_url

Uri#port doesn't take a string anymore

Change-Id: Ieb0c0bf27f6e29bbe5c7d6acfefaa12c58a4ede7
Reviewed-on: https://gerrit.instructure.com/29757
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Liz Abinante <labinante@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>
QA-Review: Cameron Matheson <cameron@instructure.com>