fields.py updated to make it compatible with Django 1.7

This was necessary due to the fact that ``init()`` method from ``forms.fields.Field`` takes no "max_length" paramater. Yet, the ``formfield()`` method from ``models.TextField`` for ``RichTextField`` was adding this parameter (check out line 1853 in ``django.db.models.fields.__init__.py``).

Replacing ``models.TextField`` with ``models.Field`` made this issue disappear.

Tested with django-1.7b1