Automatically retry up to 3 times on failure when installing
    Chocolatey modules, using the 'appveyor-retry' script.  This can be
    removed later if needed.

    Sometimes Chocolatey returns a 404 for a package that exists.  This
    happens a lot.  The official fix is to get the Business edition for
    the more-stable private CDN, but since we're an open-source community
    effort, sometimes a little kludgery goes a long way.

    See http://help.appveyor.com/discussions/suggestions/816-generic-wrapper-for-retry#comment_40579488
    And appveyor/ci#418