Increase Django, Haystack and Whoosh minimum versions

Django 1.5+
Whoosh 2.5+
Haystack 2.1+

Signed-off-by: Byron Ruth <b@devel.io>
Monkey patch Haystack SearchQuerySet

This patches the SearchQuerySet.__len__ method to prevent return a
negative number of search results noted in toastdriven/django-haystack#1009

Signed-off-by: Byron Ruth <b@devel.io>