[flutter] Release 0.0.4 with breaking changes.

spine-flutter/pubspec.yaml

@@ -1,6 +1,6 @@
 name: esotericsoftware_spine_flutter
 description: The official Spine Flutter Runtime to load, display and interact with Spine animations.
-version: 0.0.3
+version: 0.0.4
 homepage: https://esotericsoftware.com
 repository: https://github.com/esotericsoftware/spine-runtimes
 issue_tracker: https://github.com/esotericsoftware/spine-runtimes/issues