Add Changelog entry to fix for removing artifacts

CHANGELOG.md

@@ -2,6 +2,8 @@ Please view this file on the master branch, on stable branches it's out of date.
 
 ## 8.13.0 (2016-10-22)
 
+v 8.13.0 (unreleased)
+  - Avoid race condition when asynchronously removing expired artifacts. (!6881)
   - Improve Merge When Build Succeeds triggers and execute on pipeline success. (!6675)
   - Respond with 404 Not Found for non-existent tags (Linus Thiel)
   - Truncate long labels with ellipsis in labels page