Update Django versions and CI link in README and documentation

[browniebroke]

Not sure if you also want to remove the fallback code for Django < 3.1:

django-modeltrans/modeltrans/fields.py

Lines 14 to 20 in 07c3dd1

	try:
	# django==3.1 moved JSONField into django.db.models
	from django.db.models import JSONField
	from django.db.models.fields.json import KeyTextTransform
	except ImportError:
	from django.contrib.postgres.fields import JSONField
	from django.contrib.postgres.fields.jsonb import KeyTextTransform

[jieter]

Not sure if you also want to remove the fallback code for Django < 3.1:

django-modeltrans/modeltrans/fields.py

Lines 14 to 20 in 07c3dd1

	try:
	# django==3.1 moved JSONField into django.db.models
	from django.db.models import JSONField
	from django.db.models.fields.json import KeyTextTransform
	except ImportError:
	from django.contrib.postgres.fields import JSONField
	from django.contrib.postgres.fields.jsonb import KeyTextTransform

Yes please!

[browniebroke]

Should be all done now

[jieter]

Looks good, thanks!