Add compatibility for Django v2

[kathomas921]

Installation instructions are break with release of Django 2.0

'pip install -U mezzanine-api' downloads Django 2.02, which no longer has the 'django.forms.extras' package, which throws an exception when running 'python manage.py createdb --noinput' in the installation instructions.

Either eliminating that dependency or adjusting the pip package to download Django 1.11 instead of the newest version should fix this (I imagine).

[gcushen]

Thanks for reporting this! The master version of Mezzanine-API should now support Django v2.