use binmode for s3 download file

test plan:
 - you should be able to perform content imports on rails3
   (specifically, there should not be a "Content-length does not match",
    or, for very small imports, "(something) from ASCII-8BIT to UTF-8"
    error)

fixes CNVS-12229

Change-Id: I8a549c3760972dc028461b391c87a2fcc98ff9f4
Reviewed-on: https://gerrit.instructure.com/33186
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>