add missing changelog entry for using image_size ~> 1.5

Author: toy

CHANGELOG.markdown

@@ -2,6 +2,7 @@
 
 ## unreleased
 
+* Use `image_size ~> 1.5` with `apng` detection, so apng images are not optimised to one frame version [#142](https://github.com/toy/image_optim/issues/142) [@toy](https://github.com/toy)
 * Don't show `?` for unknown bin version in message about inability to determine version [@toy](https://github.com/toy)
 * Deduplicate bin resolving error messages [@toy](https://github.com/toy)