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
(cherry picked from commit
19f50b66742c42f9139d34a61a3198981dfbe5ed)