ENH: upgrade django-haystack

This commit requires following
1. django-haystack==2.1.0
2. xapian-haystack==2.0.0
3. xapian-python-bindings-for-python-2.7.0==1.2.8
4. Whoosh==2.5.1

This code is NOT backward compatible with older versions of django-haystack, xapian-haystack
Kindly install xapian-haystack from its repository and NOT with `pip`
https://github.com/notanumber/xapian-haystack/ (master)