Update to Django 2.1.1

[timgraham]

Dependencies to check:

• dj-database-url https://github.com/kennethreitz/dj-database-url/pull/109
• django-classy-tags drop testing for Django < 1.11, add testing for Django 1.11, 2.0, and 2.1 django-cms/django-classy-tags#40
• django-password-reset
• django-redis-cache Add testing for Django 1.11 - 2.1 and drop testing for older versions sebleier/django-redis-cache#154
• django-sekizai Add support for Django 1.11, 2.0, 2.1, and 2.2, and drop support for older versions django-cms/django-sekizai#79
• django-ses Drop support for Django < 1.11, and add testing for Django 2.1 django-ses/django-ses#160
• django-storages (version after 1.6.6 will be tested with Django 2.1)
• django-tagging Add testing for Django 1.11, 2.0, 2.1, and drop testing for older versions Fantomas42/django-tagging#18