Update gunicorn to support dynamic configuration reload

This PR bumps gunicorn from 18.0.0 to 19.1.1.  With the new version, the controller now properly reloads settings when a `SIGHUP` signal is received.  This is important for upcoming work on controller HA in case the database or registry move hosts.

Closes #1928.
Refs #984.
This is specific to gunicorn. You only get django access logs when using their dev server.