add dart2 support

pubspec.yaml

@@ -1,12 +1,12 @@
 name: webfeed
-version: 0.2.1
+version: 0.2.2
 description: A dart package for parsing RSS and Atom feed.
 author: Wito Chandra <wito.c.91@gmail.com>
 homepage: https://github.com/witochandra/webfeed
 environment:
-  sdk: ">=1.8.0 <2.0.0"
+  sdk: ">=1.8.0 <3.0.0"
 dependencies:
   xml: "^3.0.0"
 dev_dependencies:
-  test: ^0.12.0
+  test: ^1.3.0
   http: "^0.11.3+16"