Fix forms issue in Django 1.6+.

Fix forms issue in Django 1.6+.

In Django 1.6, you must explicitly declare that you want to use all
fields from the model on a ModelForm.  Prior to Django 1.6, this was
not necessary.

Fixes the following errors:

    ImproperlyConfigured: Creating a ModelForm without either the
    'fields' attribute or the 'exclude' attribute is prohibited; form
    MembershipAdminForm needs updating.

    ImproperlyConfigured: Creating a ModelForm without either the
    'fields' attribute or the 'exclude' attribute is prohibited; form
    UserProfileAdminForm needs updating.

Signed-off-by: Jason Stephenson <jason@sigio.com>