Latest PowerShell task SDK is incompatible with ClientOM 19 and 20

[jessehouwing]

This causes the following function:

Get-TfsClientCredentials

to fail, because the class Microsoft.TeamFoundation.Client.TfsClientCredentials no longer exists.

This has been replaced by Microsoft.VisualStudio.Services.Client.VssClientCredentials from assembly: Microsoft.VisualStudio.Services.Client.Interactive.

The Get-TfsClientCredentials should probably test for the existence of the VssClientCredentials and use that, then fall back to TfsClientCredentials for backwards compat.