Tests: use/add Django 1.7.x, Python 3.4

Also fix matrix exclusion with travis and remove '-q' with pip-install /
setup.py.
Update Django 1.7 source, add pypy/pypy3
travis: skip Django 1.4 with pypy3, sort tox.ini
Do not blindly override select_related or prefetch_related.

When empty `select_related` or `prefetch_related` attributes are given
to subclasses of `SelectRelatedMixin` and `PrefetchRelatedMixin`, the
original query set should be kept: it may already have its own
select_related or prefetch_related.
Merge pull request #149 from Polyconseil/fix/dont_override_select_related_from_queryset

Do not blindly override select_related or prefetch_related.
Merge pull request #147 from blueyed/test-django-1.7c2

Tests: use Django 1.7, fix/improve travis.yml