Add missing Pygments documentation build dependency.
Update gunicorn to 19.1.0 and use a Python module for configuration.
Introduce jitter into gunicorn's max_requests setting.

As the worker processes get an almost equal number of requests, they
where recycled at almost the same time, leading to spikes in the
95 percentile response times.
Switch pygeoip module to use memory caching.

This avoids problems in changing the data file underneath the running
process.