Comma missing in MIDDLEWARE_CLASSES breaks things for Django <2

[code-review-doctor]

The lack of comma here on line 43:

django-colorfield/tests/settings.py

Lines 27 to 32 in 5e465af

	MIDDLEWARE_CLASSES = [
	'django.contrib.auth.middleware.AuthenticationMiddleware',
	'django.contrib.messages.middleware.MessageMiddleware'
	'django.contrib.sessions.middleware.SessionMiddleware',
	'django.middleware.common.CommonMiddleware',
	]

results in implicit string concatenation, so 'django.contrib.messages.middleware.MessageMiddleware' will get concatenated with the next line 'django.contrib.sessions.middleware.SessionMiddleware', resulting in Django attempting to import a class from 'django.contrib.messages.middleware.MessageMiddlewaredjango.contrib.sessions.middleware.SessionMiddleware', which will of course fail.

It looks like Django <2.0 is still supported so I think this should be fixed

I can make a PR for you :)