bump to 0.5.4

pubspec.yaml

 name: fluwx
 description: A implement of WeChatSDK on Flutter.Fluwx makes possible using WeChatSDK in Flutter.Enjoy sharing or payment in Flutter.
-version: 0.5.3
+version: 0.5.4
 author: JarvanMo <jarvan.mo@gmail.com>
 homepage: https://github.com/JarvanMo/fluwx