[REEF-1861] Update NuGet feed to v3

  Resolving nuget restore failure on some CI builds

JIRA:
  [REEF-1861](https://issues.apache.org/jira/browse/REEF-1861)

Pull request:
  This closes #1362
diff --git a/lang/cs/nuget.config b/lang/cs/nuget.config
index 79317f3..a8f428a 100644
--- a/lang/cs/nuget.config
+++ b/lang/cs/nuget.config
@@ -19,6 +19,7 @@
 -->
 <configuration>
     <packageSources>
+        <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
         <add key="nuget.org" value="https://www.nuget.org/api/V2" />
         <add key="dotnet" value="https://dotnet.myget.org/F/dotnet-core/api/v3/index.json" />
     </packageSources>