Make it work on Mono.

Specifically, Mono 3.2.8 on Ubuntu 14.04, which hasn't yet implemented
WebRequest.CreateHttp() and gets confused with version numbers for
System.Web.Http.

It seems that Linq to Twitter forces version 1.5.0.0 for System.Web.Http,
but Mono/Ubuntu only gives us version 4.0.0.0.