Update changelog

Author: cmd-ntrf

CHANGELOG

@@ -3,6 +3,11 @@ All notable changes to this project will be documented in this file.
 
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
+## [6.4] - 2020-03-11
+
+### Changed
+- [cloud-init] Hardcoded the version of puppet-agent (6.13.0) and puppetserver (6.9.1).
+
 ## [6.3] - 2020-03-09
 
 ### Added