Update changelog: unreleased -> v0.5.5 (#48)

Author: bsweger

CHANGELOG.md

@@ -5,7 +5,7 @@ All notable changes to `pyprefab` are documented here.
 The format is based on [Keep a Changelog](https://keepachangelog.com), and the
 project uses [Semantic Versioning](https://semver.org/).
 
-## Unreleased
+## 0.5.5
 
 ### Added