Remove hidden call to unsafe https://jet.geex-arts.com/ping.gif 1x1 pixel image

[tuergeist]

This hidden and undocumented call to an image shall be removed. I discovered it when configuring Content Security Policy directives.

AND, the certificate used by that domain is invalid as well.

[fannigurt]

https://github.com/geex-arts/django-jet/issues/260