Add support for Django 1.8

[dominicrodger]

django.test.simple is no more, so django-setuptest doesn't support Django 1.8, since django-setuptest imports from there in setuptest.py.

I'm not entirely sure what build_suite and build_test do, but it looks like we'll need to find equivalents for them in Django 1.8. There's a method called build_suite in django.test.runner, in the class DiscoverRunner, but it's not obvious to me that it does the same thing.

[pcraston]

:+1:

[cancan101]

:+1:

[paulheideman]

:+1:

[bashu]

+1

[stellasia]

:+1:

[lamby]

This can be closed now.

[lamby]

(See also https://github.com/praekelt/django-setuptest/pull/26)