[system.net.http] Set version to 4.0.0.0 on MOBILE

For a variety of reasons, we need to bump the assembly version of
System.Net.Http.dll to "4.0.0.0" (from 2.0.5.0) on our MOBILE
profiles, lest much pain and suffering occur, e.g.:

	http://motzcod.es/post/78863496592/portable-class-libraries-httpclient-so-happy

> For some reason at compile time the facades don’t kick in
> properly and it tries to use the NuGet version of
> System.Net.Http which has some specific functionality for
> older platforms. ...
> All you need to do is create a new app.config file in your
> Android and iOS project.

The need to create a custom app.config is ugly and something
to be avoided. Setting our System.Net.Http.dll version to
v4.0.0.0 removes the need for the custom app.config.