Ensure that we reference System.Net.Http 4.0.0.0

[jcansdale]

This is a tracking issue for #2188 #2186 #2185.

The primary symptom is:

System.MissingMethodException: Method not found:
'System.Net.Http.HttpMessageHandler Octokit.Internal.HttpMessageHandlerFactory.CreateDefault()'.
at GitHub.Infrastructure.ExportedHttpClient..ctor()

See the following for more info:

• https://github.com/dotnet/corefx/issues/17522#issuecomment-296868184
• https://github.com/dotnet/corefx/issues/17522#issuecomment-338418610