properly handle timezone offsets in case updates

*I only tested by running the tests and doing a basic sanity check of the function I added in dimagi-utils*

http://manage.dimagi.com/default.asp?66970

modified_on, etc. currently get stripped of their timezone info without
adjusting the time to UTC.  This fixes that for future submissions.

Depends on https://github.com/dimagi/dimagi-utils/pull/125