Prepare release

packages/zefyr/pubspec.yaml

 name: zefyr
 description: Clean, minimalistic and collaboration-ready rich text editor for Flutter.
-version: 0.3.0
+version: 0.3.1
 author: Anatoly Pulyaevskiy <anatoly.pulyaevskiy@gmail.com>
 homepage: https://github.com/memspace/zefyr