pngjs version is out of date

[benjie]

The latest pngjs supports node >= v4;

https://github.com/lukeapage/pngjs/blob/b495e60323125cd3f2dcd64c56fa0182b9b49c65/package.json#L30

The version pngjs-image is using requires node v0.8:

https://github.com/lukeapage/pngjs/blob/82dd9a96c00eabb2b9051edb6d7b45fed944111d/package.json#L12

Please upgrade pngjs to a newer version so that it can be installed under the yarn package manager on new node.js engines, if possible.

(I ❤️ blink-diff yahoo/blink-diff#32)