Temporary fix for docfx
Temporary fix for docfx

- Update to latest docfx; this doesn't solve the problem,
  but means we can assume that if the directory is there, the
  user has already run *this* version of the script.
- Copy MSBuild DLLs into the docfx directory as per
  https://github.com/dotnet/docfx/issues/2491#issuecomment-371739979

It's possible that if a user runs this as part of a build and *then*
updates to VS15.6, they may need to delete the packages directory
and let the magic happen again.

Fixes #1969.

jskeet/google-cloud-dotnet,iantalarico/google-cloud-dotnet,googleapis/google-cloud-dotnet,evildour/google-cloud-dotnet,jskeet/google-cloud-dotnet,jskeet/google-cloud-dotnet,jskeet/google-cloud-dotnet,iantalarico/google-cloud-dotnet,jskeet/gcloud-dotnet,jskeet/google-cloud-dotnet,googleapis/google-cloud-dotnet,evildour/google-cloud-dotnet,evildour/google-cloud-dotnet,googleapis/google-cloud-dotnet,iantalarico/google-cloud-dotnet