Modify widgets.py to conditionally define _has_changed depending on Django version

Django 1.6 changed how the _has_changed method is defined, moving it from widgets to form fields. See: https://docs.djangoproject.com/en/dev/releases/1.6/#has-changed-method-on-widgets

Subsequently, the SelectMultiple widget now breaks when submitted in a form in Django 1.6+. The attached change conditionally sets that method on the widget based on the Django version.