image-width/height support for SVG with only viewBox

[rvock]

Currentlyimage-width or image-height does not work on SVG images, which are missing width and height attributes and only have a viewBox.

There is an upstream fix for this issue: image-size/image-size#38

You just need to adjust the dependency for it. ^0.3.5 does not get 0.6.3.