Add Changelog entry for undefined method fix in commit builds

CHANGELOG

@@ -9,6 +9,9 @@ v 8.5.0 (unreleased)
   - Whitelist raw "abbr" elements when parsing Markdown (Benedict Etzel)
   - Don't vendor minified JS
 
+v 8.4.2 (unreleased)
+  - Fix method undefined when using external commit status in builds
+
 v 8.4.1
   - Apply security updates for Rails (4.2.5.1), rails-html-sanitizer (1.0.3),
     and Nokogiri (1.6.7.2)