Profile78 => Profile259, which is the same profile without WCF

In Reactive-Extensions/Rx.NET#4, we enabled Rx.NET for Universal Apps (i.e. *just* Win81 + WP81). However, the CLR team also introduced a new *Portable* profile for WPA81, Profile259. This profile is like Profile78 (the good 4.5 Profile), but adds support for WP81 by removing WCF. Why they couldn't just implement WCF on Windows Phone I'll never know, but it is what it is. 

Anyways, in the NuGet package builder, you need to **straight-up copy** the `lib\Portable-Net45+WinRT45+WP8` into a `lib\Portable-Net45+WinRT45+WP8+WPA81` folder. It'll all work. I think. 

/cc @jlaanstra