fix tempfile_for_uri for rails 3

the Tempfile doesn't default to binary in rails 3

Change-Id: I3478f97b8c330c770e517fb6618695fc20cab990
Reviewed-on: https://gerrit.instructure.com/30699
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>