Use GenericIPAddressField

[benmurden]

This fixes the following warning.

WARNINGS:
billing.PinCharge.ip_address: (fields.W900) IPAddressField has been deprecated. Support for it (except in historical migrations) will be removed in Django 1.9.
        HINT: Use GenericIPAddressField instead.
billing.WorldPayResponse.ip_address: (fields.W900) IPAddressField has been deprecated. Support for it (except in historical migrations) will be removed in Django 1.9.
        HINT: Use GenericIPAddressField instead.

https://docs.djangoproject.com/en/1.9/ref/models/fields/#django.db.models.GenericIPAddressField