Update library reference to reflect changes made in Django 1.6

The update_wrapper function import was removed in Django 1.6.

Reference: https://github.com/kezabelle/django-haystackbrowser/issues/5
Code: https://github.com/django/django/commit/876fc3912881e15eda2b02c3b4933af313d0a498
Documentation: https://docs.python.org/2/library/functools.html#functools.update_wrapper