Update INSTALL_REQUIRES in setup.py

Since `PIL` is imported in `html.py`, it should be pulled in during installation time in order to avoid import errors. `PIL` (specifically it's newer incarnation `Pillow`) is optional for `django-cms` but here it looks like it's mandatory. It's also nice to check for Django CMS during installation time because `djangocms-text-ckeditor` is a Django CMS plugin.