[DEPENDENCIES] Use latest gunicorn.
Patch gevent.pywsgi.WSGIHandler in inbox.api.srv.

This way it will work properly when run from gunicorn (e.g. under
supervisor) as well as when run from bin/inbox-api (for development).
[DEPENDENCIES] Don't install supervisor from PyPI.

This just doesn't seem to work on Debian/Ubuntu; use the version from
APT instead, which we already install in setup.sh. If you are attempting
to run a proper prod setup, you will wish to 'pip uninstall supervisor'.
(If this doesn't work, you'll have to manually remove the files in
/usr/local.)