OPENNLP-1305: Disable DTD loading. (#376)

Author: jzonthemtn

opennlp-tools/src/main/java/opennlp/tools/formats/letsmt/LetsmtDocument.java

@@ -124,6 +124,7 @@ static LetsmtDocument parse(InputStream letsmtXmlIn) throws IOException {
       XMLReader xmlReader = saxParser.getXMLReader();
       LetsmtDocumentHandler docHandler = new LetsmtDocumentHandler();
       xmlReader.setContentHandler(docHandler);
+      xmlReader.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true);
       xmlReader.parse(new InputSource(letsmtXmlIn));
       return new LetsmtDocument(docHandler.sentences);
     } catch (SAXException e) {