Fix NuGet restore for AppVeyor.
THis CI server does not natively support the restored packages, so the
Microsoft.Bcl.Build must be included into the source control.
For more info see this: http://stackoverflow.com/a/20016486/430254
And this: http://support.appharbor.com/discussions/problems/37286-the-build-restored-nuget-packages-build-the-project-again-to-include-these-packages-in-the-build-for-more-information-see-httpgomicrosoftcomfwlinklinkid317568