Update image_size 1.2.0 -> 1.3.0

The only new thing in this version is that image_size will raise
FormatError instead of RuntimeError[1]. We can then rescue this
explicitly when using ImageSize.

[1]: https://github.com/toy/image_size/commit/8dfeab767ffd83f4b0e965886ef8310eee19ea2d
Merge branch 'update-image_size'