Update to latest xunit with appveyor reporter improvements

Rx.NET/Source/NuGet.Config

@@ -1,6 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <configuration>
   <packageSources>
+    <add key="xUnit CI" value="https://www.myget.org/F/xunit/api/v3/index.json" />
     <add key="Build Packages" value="https://www.myget.org/F/c037199d-41df-4567-b966-25ff65324688/api/v3/index.json" />
     <add key="NuGet.org" value="https://api.nuget.org/v3/index.json" />
   </packageSources>

Rx.NET/Source/Tests.System.Reactive/project.json

@@ -30,8 +30,8 @@
     "System.Reactive.Experimental": { "target": "project" },
     "System.Reactive.Providers": { "target": "project" },
     "System.Reactive.Observable.Aliases": { "target": "project" },
-    "xunit": "2.2.0-beta2-build3300",
-    "dotnet-test-xunit": "2.2.0-preview2-build1029",
+    "xunit": "2.2.0-beta3-build3340",
+    "dotnet-test-xunit": "2.2.0-preview3-build1035",
     "System.Reflection.TypeExtensions": "4.1.0",
     "System.Diagnostics.StackTrace": "4.0.1"
   },