Don't restrict user model name and fix travis python3 build

Your changes seemed to work ok when testing locally. I'd just like to propose this small change to permit the custom user model to be named anything, not just "User".

Travis' python 3 build also broke due to wrong syntax with relative import. It's fixed in the other commit.

I enabled Travis CI for my local fork, so this change can be seen at https://travis-ci.org/koirikivi/django-cms/builds/19685673

Thanks!