Should throw an error if django-compressor version is < 1.3

[rockymeza]

django-compressor==1.3 introduced the filter feature that django_pyscss.compressor.DjangoScssFilter relies on. We can't just put it in install_requires in the setup.py because some people might want to use django-pyscss without compressor.

[rockymeza]

We should also throw an error if pyscss is too old.