nuget package fixes

* Updated all newtonsoft.json versions to 6.0.3.
* Updated TestClient rx-main version to 1.0.11226 - now on par with AustinHarris.JsonRpc.Client.
* Removed hard-linked Newtonsoft.Json.dll from the root folder - we should be just using nuget package now.
* Removed hard-linked System.Reactive.dll from root folder - we should be just using nuget package now.
* Fixed solution file - TestServer.csproj should load all good now.

Fixes #18.
Fixed #16.