Made the ping_google command override BaseCommand.handle() rather than execute().

Overriding execute() bypasses the system check framework among other things.

django/contrib/sitemaps/management/commands/ping_google.py

@@ -8,5 +8,5 @@ class Command(BaseCommand):
     def add_arguments(self, parser):
         parser.add_argument('sitemap_url', nargs='?', default=None)
 
-    def execute(self, *args, **options):
+    def handle(self, *args, **options):
         ping_google(sitemap_url=options['sitemap_url'])