Django 1.9 compatibility

[marcinn]

Package will not work in Django 1.9 due to deprecated import in forms:

`django_json_field-0.5.7-py2.7.egg/json_field/forms.py:5: RemovedInDjango19Warning: The django.forms.util module has been renamed. Use django.forms.utils instead.``

There is no necessary to import django.forms.util(s), because you're using it to raise ValidationError. Just import forms and change util.ValidationError to forms.ValidationError, or import ValidationError from django.core.exceptions.

[w-]

there is already a PR i've submitted for this. just waiting for it to be accepted ='(
#41