CI: Add “appveyor-retry” to retry download binary (#525)
CI: Add “appveyor-retry” to retry download binary (#525)

Sometime, it fails to download ImageMagick installer binary on AppVeyor.

I don't like to restart CI build by download failure.

So, this PR introduce retry command.
https://github.com/appveyor/ci/blob/master/scripts/appveyor-retry.cmd
rmagick/rmagick,rmagick/rmagick,rmagick/rmagick