hotfix: show the 'balance' field when creating a new member

For an unknown reason, the default value of the 'balance' field isn't used when we create a member if the 'balance' field isn't displayed.

This fix just show the 'balance' field.

coin/members/admin.py

@@ -88,7 +88,9 @@ class MemberAdmin(UserAdmin):
             'type',
             ('first_name', 'last_name', 'nickname'),
             'organization_name',
-            'comments')}),
+            'comments',
+            'balance' # XXX we shouldn't need this, the default value should be used
+        )}),
         ('Coordonnées', {'fields': (
             'email',
             ('home_phone_number', 'mobile_phone_number'),