title: NuGet install-package failing
body: <p>I'm attempting to install xunit version 2.1.0.. However I'm getting the following exception:</p>
  
  <p>Install-Package : Exception 'System.AggregateException' thrown when trying to add source '<a href="https://api.nuget.org/v3/index.json" rel="noreferrer">https://api.nuget.org/v3/index.json</a>'. Please verify all your o
  nline package sources are available.
  At line:1 char:16
  + Install-Package &lt;&lt;&lt;&lt;  xunit -Version 2.1.0
      + CategoryInfo          : NotSpecified: (:) [Install-Package], Exception
      + FullyQualifiedErrorId : NuGetCmdletUnhandledException,NuGet.PackageManagement.PowerShellCmdlets.InstallPackageCommand</p>
  
  <p>Any ideas on whats going wrong? Sadly google is not giving me anything helpful..</p>