libravatar.org supports HTTPS (#3498)

conf/app.ini

@@ -257,7 +257,7 @@ GRAVATAR_SOURCE = gravatar
 ; This value will be forced to be true in offline mode.
 DISABLE_GRAVATAR = false
 ; Federated avatar lookup uses DNS to discover avatar associated
-; with emails, see http://www.libravatar.org
+; with emails, see https://www.libravatar.org
 ; This value will be forced to be false in offline mode or Gravatar is disbaled.
 ENABLE_FEDERATED_AVATAR = false