django.core.management.execute_manager is deprecated

@kumar303 @andymckay @robhudson r?

This change makes the deprecation warning go away when using funfactory in django 1.5. 
tox still passes. 
I tested this in my airmozilla project with Django 1.6 and it worked. 

Also, I checked that you can still override stuff on the command line. E.g. `./manage.py shell --settings=airmozilla.settings_copy` and it picked it up correctly.