Update README and CHANGELOG for release 5.1.1 (#171)

Author: mokagio

CHANGELOG.md

@@ -42,12 +42,18 @@ _None._
 
 ### Bug Fixes
 
-- Make `install_gems` account for different C system libraries if `Gemfile.lock` includes `nokogiri` [#149]
+_None._
 
 ### Internal Changes
 
 _None._
 
+## 5.1.1
+
+### Bug Fixes
+
+- Make `install_gems` account for different C system libraries if `Gemfile.lock` includes `nokogiri` [#149]
+
 ## 5.1.0
 
 ### New Features

README.md

@@ -26,11 +26,11 @@ steps:
       restore_cache $(hash_file package-lock.json)
 
     plugins:
-      - automattic/a8c-ci-toolkit#5.1.0:
+      - automattic/a8c-ci-toolkit#5.1.1:
           bucket: a8c-ci-cache # optional
 ```
 
-Don't forget to verify what [the latest release](https://github.com/Automattic/a8c-ci-toolkit-buildkite-plugin/releases/latest) is and use that value instead of `5.1.0`.
+Don't forget to verify what [the latest release](https://github.com/Automattic/a8c-ci-toolkit-buildkite-plugin/releases/latest) is and use that value instead of `5.1.1`.
 
 ## Configuration