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
I had same problem and reinstalling package doesn't work.
My application runs in LocalIIS(not IIS EXPRESS). I enable Assembly Load Trace and find out it want load System.Web.Http load from below temp catche. so remove all catch folders in it.
C:\Windows\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files[my application]