Migrate to dotnet.myget.org feed

NuGet.config

@@ -1,7 +1,7 @@
-<?xml version="1.0" encoding="utf-8"?>
+<?xml version="1.0" encoding="utf-8"?>
 <configuration>
   <packageSources>
-    <add key="AspNetVNext" value="https://www.myget.org/F/aspnetcidev/api/v3/index.json" />
+    <add key="AspNetCore" value="https://dotnet.myget.org/F/aspnetcore-ci-dev/api/v3/index.json" />
     <add key="NuGet" value="https://api.nuget.org/v3/index.json" />
   </packageSources>
 </configuration>