Slaprunner (gunicorn): version up

I have been facing some gunicorn issues while fixing tickets. The gunicorn fails start because the pid has been taken by some other process.
This bug has been solved in the v19.7.0.
See: https://github.com/benoitc/gunicorn/blob/master/gunicorn/pidfile.py#L75
Also See: https://github.com/benoitc/gunicorn/releases/tag/19.7.0

Almsost a year ago Nicolas fixed the version of gunicorn to v19.4.5 here 391b026e
Since then the bug stoping up from upgrading has been fixed as well.

/reviewed-on nexedi/slapos!228