For anyone landing here who is trying to upgrade from MVC 4 to MVC5, I was able to resolve this issue by following the instructions at 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 also had to install the "Microsoft.AspNet.WebApi.WebHost" package from nuget. But that's it.
Oh, and I had to create this appSetting: <add key="owin:AutomaticAppStartup" value="false" />
:)