Remove usage of forms.ALL_FIELDS

Doesn't work in django 1.5:

    Traceback (most recent call last):
      File "local/lib/python2.7/site-packages/django/core/handlers/base.py", line 90, in get_response
        response = middleware_method(request)
      File "local/lib/python2.7/site-packages/django/middleware/locale.py", line 21, in process_request
        check_path = self.is_language_prefix_patterns_used()
      File "local/lib/python2.7/site-packages/django/middleware/locale.py", line 56, in is_language_prefix_patterns_used
        for url_pattern in get_resolver(None).url_patterns:
      File "local/lib/python2.7/site-packages/django/core/urlresolvers.py", line 366, in url_patterns
        patterns = getattr(self.urlconf_module, "urlpatterns", self.urlconf_module)
      File "local/lib/python2.7/site-packages/django/core/urlresolvers.py", line 361, in urlconf_module
        self._urlconf_module = import_module(self.urlconf_name)
      File "local/lib/python2.7/site-packages/django/utils/importlib.py", line 35, in import_module
        __import__(name)
      File "/media/archive/documents/code/skifergas/skifergas/urls.py", line 9, in <module>
        admin.autodiscover()
      File "local/lib/python2.7/site-packages/django/contrib/admin/__init__.py", line 29, in autodiscover
        import_module('%s.admin' % app)
      File "local/lib/python2.7/site-packages/django/utils/importlib.py", line 35, in import_module
        __import__(name)
      File "local/lib/python2.7/site-packages/zinnia/admin/__init__.py", line 6, in <module>
        from zinnia.admin.entry import EntryAdmin
      File "local/lib/python2.7/site-packages/zinnia/admin/entry.py", line 25, in <module>
        from zinnia.admin.forms import EntryAdminForm
      File "local/lib/python2.7/site-packages/zinnia/admin/forms.py", line 52, in <module>
        class EntryAdminForm(forms.ModelForm):
      File "local/lib/python2.7/site-packages/zinnia/admin/forms.py", line 69, in EntryAdminForm
        class Meta:
      File "local/lib/python2.7/site-packages/zinnia/admin/forms.py", line 74, in Meta
        fields = forms.ALL_FIELDS
    AttributeError: 'module' object has no attribute 'ALL_FIELDS'