Revert "Swap back to nuget.org v2 feed"

This reverts commit a17da51.

Author: rynowak

NuGet.Config

@@ -2,6 +2,6 @@
 <configuration>
   <packageSources>
     <add key="AspNetVNext" value="https://www.myget.org/F/aspnetlitedev/api/v2" />
-    <add key="NuGet" value="https://www.nuget.org/api/v2/" />
+    <add key="NuGet" value="https://api.nuget.org/v3/index.json" />
   </packageSources>
 </configuration>