Update django-cas Version

Update django-cas Version .

As the old mitx version can not support  CAS 2.0 and CAS 3.0 provided by ja-sig.org(java) 
So I open this pull.

eduStack/django-cas is a fork of https://bitbucket.org/cpcc/django-cas which use hg as version control.

cas_mapper_example:  https://github.com/eduStack/bistux_cas_mapper