Fix compatibility with Django < 1.8
Fix compatibility with Django < 1.8

The 1.6.1 release added a dependency on the `setting_changed` signal. In Django
releases prior to 1.8, this signal is located in the `django.test.signals`
module, and in 1.8, an alias was added in `django.core.signals`.

This change adds a fallback import to restore compatibility with older versions
for applications which cannot (yet) migrate to the newer Djangos. It has been
tested against Django 1.6.11 and 1.8.9.

lexqt/django-pipeline,d9pouces/django-pipeline,sideffect0/django-pipeline,beedesk/django-pipeline,cyberdelia/django-pipeline,chipx86/django-pipeline,d9pouces/django-pipeline,d9pouces/django-pipeline,theatlantic/django-pipeline,sideffect0/django-pipeline,cyberdelia/django-pipeline,beedesk/django-pipeline,theatlantic/django-pipeline,sideffect0/django-pipeline,kronion/django-pipeline,chipx86/django-pipeline,jazzband/django-pipeline,jazzband/django-pipeline,jazzband/django-pipeline,chipx86/django-pipeline,kronion/django-pipeline,cyberdelia/django-pipeline,kronion/django-pipeline,beedesk/django-pipeline,theatlantic/django-pipeline,lexqt/django-pipeline,lexqt/django-pipeline