Retry Chocolatey install to workaround failures

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 https://github.com/appveyor/ci/issues/418