Bump 'image-min' dependency to v0.1.3

When using Windows:

`optipng.exe: downloading [====               ] 25% 0.9s✗ pre-build test failed, compiling from source...
✗ building is not supported on win32`

The latest 'image-min' is v0.1.3 does away with the dependency `optipng-bin` in favour of an optional dependency `pngquant-bin`

`grunt-contrib-imagemin` currently has a dependency of `image-min` v0.1.1
* `v~0.1.1` has a dependency on `optipng-bin`
* `v~0.1.2` has `optipng-bin` as an optional dependency 
* `v~0.1.3` removes the `optipng-bin` dependency in favour of `pngquant-bin`

So in theory this should fix this error