Require django-compressor>=1.3 for django-pyscss.

Django-compressor 1.3 introduces the filter class functionality that
django-pyscss relies on.  django-pyscss doesn't require that version of
django-compressor because it is compressor agnostic.  It might be nice
to add an error into django-pyscss if django-compressor < 1.3 is
installed though.