Revert to using MVC 4.0.0.0 OOB so that people who have only 4.0.0.0 in the GAC can actually install Umbraco. If 4.0.0.1 is in the GAC that version will be taken.

Leaves the additional reference to System.Web.Http in web.config so that you don't get the System.Web.Http.ApiController when 4.0.0.1 is indeed in the GAC.