When we migrated run-dev.py from Twisted to Tornado a few weeks ago,
the --interface argument wasn't properly ported and thus was ignored.
This restores the original functionality of defaulting to only
listening on localhost.
Ideally, we'd replace the vagrant/zulipdev user check with something
that just checks whether a special file that is created by the
Vagrant/remote-dev-vm creation process exists; that would be more
robust.