Update travis ci testing to use Django 3.1 and Ubuntu 20.04
Update travis ci testing to use Django 3.1 and Ubuntu 20.04

rossp/django-helpdesk,rossp/django-helpdesk,rossp/django-helpdesk,django-helpdesk/django-helpdesk,gwasser/django-helpdesk,gwasser/django-helpdesk,django-helpdesk/django-helpdesk,gwasser/django-helpdesk,rossp/django-helpdesk,gwasser/django-helpdesk,django-helpdesk/django-helpdesk,django-helpdesk/django-helpdesk