Just assume django-haystack uses semver for now.

pytest-warnings is now part of pytest core. See https://docs.pytest.org/en/latest/changelog.html#id71

- Drop support for Django 1.9 (EOL)
- Drop support for Python 3.6 with Django 1.8 to 1.10

travis: Run py.test --cov=oscar instead of coverage run

This also updates the pytest dependencies

As per [their blog post of the 27th April](https://blog.readthedocs.com/securing-subdomains/) ‘Securing subdomains’:

> Starting today, Read the Docs will start hosting projects from subdomains on the domain readthedocs.io, instead of on readthedocs.org. This change addresses some security concerns around site cookies while hosting user generated data on the same domain as our dashboard.

Test Plan: Manually visited all the links I’ve modified.

Recent babel releases contain several bugfixes.

We only support two versions of Django. That policy might change given
that Django has started doing LTS releases. But for now, it only makes
sense to remove Django 1.7 support.

These are `pillow`, `phonenumbers`, `mock` and `factory_boy`.

This also reverts b1477bf

django-compressor served it's purpose, but given the advent of
more and more frontend JS work, and the accompanying build tools (grunt,
gulp, etc.), it makes less and less sense to use compressor. Jannis
Leidel himself also stepped back from maintaining it.

In my experience with Oscar project, it has also become a bit of a
liability to have compressor as a dependency. Most projects don't use it
any more, but it's not easy to disable/override because all our
templates include it.

So, let's keep things simple and remove it as a dependency. That paves
the way for using other tools, while it's always easy to add back.

See #1791