Bump mini_magick to 4.2.10

[dawilster]

Bump mini_magick to 4.2.10 fix this issue I've been having minimagick/minimagick#279

I'm putting up a PR now

[dawilster]

Sorry about this. I didn't realize that mini_magick 4.2.8 had been yanked by the author. The latest compatitable minor version 4.2.10

[hhff]

@dawilster could you submit a PR for loosening the dep like ~> 4.2.10

[dawilster]

@hhff will do