Set chunky_png dependency to 1.3.1

chunky_png 1.3.1 contains a performance improvement that will benefit
this project when saving PNG files[1] and a new method,
`euclidean_distance_rgba`[2] that we can use when computing pixel diff
scores. This has also been added to oily_png[3] and will be coming to
the next release soon.

[1]: https://github.com/wvanbergen/chunky_png/commit/5b6c205533e02221c925160bafea8801631c3b0d
[2]: https://github.com/wvanbergen/chunky_png/commit/6e350b2f4a56fb3d07cf2960da51c49a389de1dc
[3]: https://github.com/wvanbergen/oily_png/commit/341beb63d9c465a4a0baa53a3af99b700b1c50ae