Django 1.9 compatibility fixes

[smlz]

Some commits that makes django-userena work with Django 1.9.

Django 1.4 compatibility had to be dropped ({% load url from future %} was removed from Django 1.9).

So far, the changes seem to work fine, but some more testing is certainly needed before making a release.

[swistakm]

Great. Thank you very much for contribution. I will need some time to make an in-depth review but this is great that you found time to make such upgrade. First things that I noticed are:

• .travis.yml tox environments for django1.4 should be removed too to make CI tests pass
• we could remove any other django1.4 compatibility quirks (see compat.py module)

Since we are dropping django 1.4 I will have to prepare new userena==2.0.0 release and this will go there. I'm also not familiar with django1.9 yet so this will take some more time because I'm concerned about migrations.

[smlz]

Ok, I fixed the coverage and travis configs, and removed some obsolete compatibility code.

I'd probably go with 1.5. But that's just a matter of taste.

Anyway, I also had look at some DeprecationWarnings for things that are going to be removed with the next LTS release of Django (see https://www.djangoproject.com/weblog/2015/jun/25/roadmap/). With these changes your planned upcoming release would stay compatible with Django up to version 1.11. Theoretically ;-)

[swistakm]

I have merged it to v2.0.x branch because I would like to introduce some other improvements that will break backwards compatibility in 2.0.0 version before first release.

[ammsa]

Sorry to jump in but any idea when will the v2.0.x version will be released?

[swistakm]

I will try to do it in week or two because I need to do some code cleanup and I have very limited time for this project.

[swistakm]

Merged from v2.0.x branch