CHANGELOG.markdown in image_optim-0.26.3 vs CHANGELOG.markdown in image_optim-0.26.4

@@ -1,8 +1,12 @@
 # ChangeLog
 
 ## unreleased
 
+## v0.26.4 (2019-05-23)
+
+* Enable frozen string literals [@toy](https://github.com/toy)
+
 ## v0.26.3 (2018-10-13)
 
 * Handle `vnone` version of `advpng` that was erroneously produced for `ubuntu` and `homebrew` [#165](https://github.com/toy/image_optim/issues/165) [@toy](https://github.com/toy)
 
 ## v0.26.2 (2018-08-15)