Puppy Crawl no longer hosts Checkstyle DTD

(since March 2019, actually…)

Closes #118 pull request at github.com/apache/ant repo.

src/etc/checkstyle/checkstyle-config

@@ -1,5 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<!DOCTYPE module PUBLIC "-//Puppy Crawl//DTD Check Configuration 1.3//EN" "http://www.puppycrawl.com/dtds/configuration_1_3.dtd">
+<!DOCTYPE module PUBLIC
+    "-//Checkstyle//DTD Checkstyle Configuration 1.3//EN"
+    "https://checkstyle.org/dtds/configuration_1_3.dtd">
 <!--
    Licensed to the Apache Software Foundation (ASF) under one or more
    contributor license agreements.  See the NOTICE file distributed with