Update pubspec.yaml

Never published .4
diff --git a/pubspec.yaml b/pubspec.yaml
index cb55489..c841909 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: observable
-version: 0.20.5
+version: 0.20.4
 author: Dart Team <misc@dartlang.org>
 description: Support for marking objects as observable
 homepage: https://github.com/dart-lang/observable