Switched mimetype to content_type because of deprecation

The mimetype keyword was deprecated in Django 1.5,
https://docs.djangoproject.com/en/1.5/ref/request-response/#django.http.HttpResponse.__init__

The content_type keyword should be used instead.
Merge pull request #1488 from binarydud/mimetype_deprecated

Switched mimetype to content_type because of deprecation
Pin Webtest dependency

As 2.0.12 introduces a new feature which is used by an Oscar test.

We should really pin all these dependencies as using ranges is just
asking for trouble. I'll do that soon.
Fix tests for order status updating

The hidden widget means the tests have less set-up to do.
Correct typo in demo form meta
Whitespace
Install Django==1.7 from PyPi for Tox and Travis

We also drop the "allow_failures" section for Pythons 3.x on Travis.
They should pass consistently now.

I've also adjusted the 'travis' make target to not install Oscar so this
can be called separately as part of the Travis build. Hence, build
errors are reported differently from tests failing.
Remove whitespace from .travis.yml

Previous change has caused Travis to complain 'Please override the
script: key in your .travis.yml to run tests'.
https://travis-ci.org/tangentlabs/django-oscar/jobs/35319191

I am though so I'm confused. I did tinker with the whitespace last time
so here I adjust it in the hope that it fixes things. (I have no idea
what I'm doing).
Indent .travis.yml correctly

My Vim audo-indenting seems to have incorrectly put the 'script' key
under 'env'.