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

This also removes the --process-dependency-links argument to pip install
since this was only required for the unreleased version of the
django-tables package.

To support Django 1.8, we are needing a fix in tables2. It is pending a
merge in https://github.com/bradleyayers/django-tables2/pull/245

Let's hope that we'll get a release soon.

Unfortunately, this change requires that we re-enable the deprecated
--process-dependency-links to pip install. I welcome any ideas for a
better approach.

Django issued a bugfix release, so let's run against that.

* Our version of the debug toolbar threw an error, so let's bump to the
  latest release.
* django-nose only has an unreleased version with Django 1.8 support.
  I'm fully in favour of replacing it with pytest, but think it should
  happen outside of this PR. Ping @mvantellingen.
* The latest tables2 apparently contains some Django 1.8 fixes.
  Unfortunately it still breaks, but it's a step forward.

Let's see what breaks, shall we?

Vagrant support was removed from release 1.0. This commit removes mentions of it from the docs.

Closes #1559.

There was a recent security update for Pillow (to version 2.7) that this
will allow.

We hit the 1.0 milestone and generally consider it production-ready.

This allows testing against the installed package instead of the 
version controlled source.

Part of #1569

Oscar is becoming a more community-owned project. This commit reflects a
few organisational changes taking place.