Update NuGet.Config

Rx.NET/Integration/NuGet.Config

@@ -1,6 +1,7 @@
-<?xml version="1.0" encoding="utf-8"?>
+<?xml version="1.0" encoding="utf-8"?>
 <configuration>
   <packageSources>
     <add key="Local Packages" value="./LocalPackages" />
+    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
   </packageSources>
 </configuration>