lower the xml version contraints

pubspec.yaml

@@ -5,7 +5,7 @@ homepage: https://github.com/witochandra/webfeed
 environment:
   sdk: ">=2.0.0 <3.0.0"
 dependencies:
-  xml: "^4.3.0"
+  xml: "^4.2.0"
   intl: "^0.16.0"
 dev_dependencies:
   test: ^1.3.0