Enable django 2 support

[desecho]

It is still not working however because of https://github.com/KristianOellegaard/django-hvad

[erral]

Thanks for your contribution!

Can you add also this to the .travis.yml file?

We don't have an extensive test coverage, but this way we can make travis run the tests also with Django 2. I think something like this will do it:

python:
  - "2.7"
  - "3.4"
  - "3.5"
  - "3.6"

env:
  - DJANGO="Django>=1.9.0,<1.10.0"
  - DJANGO="Django>=1.10.0,<1.11.0"
  - DJANGO="Django>=1.11.0,<2.0.0"
  - DJANGO="Django>=2.0,<2.1"

[desecho]

I made the changes. However, since https://github.com/KristianOellegaard/django-hvad still does not support Django 2 it still won't work. Somebody needs to update that project as well.