Script to download the updated CA file to RubyGems on Windows will work. From the blog post: http://wragg.io/using-puppet-modules-forge-and-r10k/

fixRubyGems.ps1

$CACertFile = Join-Path -Path $ENV:AppData -ChildPath 'RubyCACert.pem'

If (-Not (Test-Path -Path $CACertFile)) {  
  "Downloading CA Cert bundle.."
  [Net.ServicePointManager]::SecurityProtocol = [Net.SecurityProtocolType]::Tls12
  Invoke-WebRequest -Uri 'https://curl.haxx.se/ca/cacert.pem' -UseBasicParsing -OutFile $CACertFile | Out-Null
}

"Setting CA Certificate store set to $CACertFile.."
$ENV:SSL_CERT_FILE = $CACertFile
[System.Environment]::SetEnvironmentVariable('SSL_CERT_FILE',$CACertFile, [System.EnvironmentVariableTarget]::Machine)