Update CHANGELOG.md

Co-Authored-By: Jacob MacDonald <jakemac@google.com>
diff --git a/CHANGELOG.md b/CHANGELOG.md
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@@ -1,4 +1,4 @@
-## 1.5.3
+## 1.5.3-dev
 
 * Improve arg parsing performance: use queues instead of lists internally to
   get linear instead of quadratic performance, which is important for large