Use protocol-relative URL for Piwik fallback img
Use protocol-relative URL for Piwik fallback img

Avoids mixed content issues when using django-analytical on sites served
over HTTPS. Using a protocol-relative URL is also recommended in the
Piwik documentation: https://issues.piwik.org/344.

Fixes #96.

jcassee/django-analytical,bittner/django-analytical,pjdelport/django-analytical