I did the upgrade according to. http://www.asp.net/mvc/tutorials/mvc-5/how-to-upgrade-an-aspnet-mvc-4-and-web-api-project-to-aspnet-mvc-5-and-web-api-2
I get the er
Or you could do this from NuGet Package Manager Console
Install-Package Microsoft.AspNet.WebApi -Version 5.0.0
And then you will be able to add the reference to System.Web.Http.WebHost 5.0