Update changelog

Author: keithduncan

CHANGELOG.md

@@ -4,6 +4,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
+## [v5.5.1](https://github.com/buildkite/elastic-ci-stack-for-aws/compare/v5.5.0...v5.5.1) (2021-08-06)
+
+### Changed
+
+* Included buildkite-agent from 3.32.0 to 3.32.1
+
 ## [v5.5.0](https://github.com/buildkite/elastic-ci-stack-for-aws/compare/v5.4.0...v5.5.0) (2021-07-30)
 
 ### Added