this problem is exactly the same as this post http://forums.asp.net/t/1807797.aspx/1?System+Net+Http+is+not+found and this one on StackOverflow
I have all the latest
I have had the same problem (couldn't build locally and remotely on azure). This workaround helped me: http://connect.microsoft.com/VisualStudio/feedback/details/760208/nuget-package-for-asp-net-mvc-4-web-api-does-not-reference-correct-net-4-5-assemblies#
here is the part you need:
Copy the System.Net.Http.dll and System.Net.Http.xml files contained in the packages\Microsoft.Net.Http.2.0.20710.0\lib\net40 directory to the packages\Microsoft.AspNet.WebApi.Core.4.0.20710.0\lib\net40 directory. Since the missing System.Net.Http.dll assembly is now in the same location as the referenced System.Web.Http.dll assembly, the code analysis can now properly resolve the conflicting System.Net.Http assembly.