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 appveyor/ci#418

scripts/build/appveyorHelp.psm1

@@ -122,7 +122,10 @@ function SETUP-QT()
 function Install-ChocolatelyModule([string] $module, [string[]] $myargs)
 {
  Write-Host "Install chocolately package $module"
- LogExec cinst $module @myargs -y
+ LogExec appveyor-retry cinst $module @myargs -y
+ # Retry installation in case it fails; remove 'appveyor-retry' to run in a generic manner
+ # See http://help.appveyor.com/discussions/suggestions/816-generic-wrapper-for-retry#comment_40579488
+ # And https://github.com/appveyor/ci/issues/418
 }
 
 function Install-CmakeGitModule([string] $url, [hashtable] $arguments)