Add missing classifiers in setup.py

[ad-m]

No description provided.

[manelclos]

Is this really needed? what's the benefit of adding specific versions?

[mgrdcm]

@manelclos I had to look myself, but it appears that the Django version classifiers are fairly commonly used: https://pypi.python.org/pypi?:action=browse&c=523

I like the idea of adding them so django-import-export can be browsed on PyPI , especially since this package supports particular versions of Django.

[bmihelac]

LGTM