Add Django 3.0 support

[tonial]

Hello,

Django 3.0 is now available.
The only two changes with this release that seems to affect django-cid are :

• dropping the django.db.backends.postgresql_psycopg2.base alias to django.db.backends.postgresql.base.
• Django 3.0 is only tested against python 3.6+

Also, Django release notes suggest to drop support for Django version prior to 2.2.

With all that in mind, I've made this PR (#51) that :

• stop testing with Django 2.1
• stop using the old alias
• Test with Django 3.0 and python 3.6-3.8

We don't need to change the requirement for Django>=1.11 django.db.backends.postgresql_psycopg2.base was already an alias to django.db.backends.postgresql.base in Django 1.11.

[dbaty]

The corresponding pull request (#51) has been merged.