The main function of prompting inside `manage.py migrate` is to ask
the user if they want to delete stale content-types, which is
unimportant and likely scary, so we disable doing so.
The #! line processing interpreted the argument to pass to `env` as
"python2.7 -u", which obviously isn't a real program.
We fix this by setting the PYTHONUNBUFFERED environment variable
inside the program, which has the same effect.
Thanks to Dan Fedele for the bug report and suggested solution!
