Add packages required for updated sources

Commands.Utility now needs Microsoft.CodeAnalysis.CSharp, which doesn't
explicitly target `dnxcore50`, so now Commands.Utility and the packages
which depend on it now much `import` the Portable Windows framework.
This also required adding the aspnetvnext feed.

System.Management.Automation now requires System.Diagnostics.StackTrace.

nuget.config

@@ -2,6 +2,7 @@
 <configuration>
   <packageSources>
     <add key="dotnetcore" value="https://www.myget.org/F/dotnet-core/" />
+    <add key="aspnetvnext" value="https://www.myget.org/F/aspnetvnext/" />
     <add key="nuget.org" value="https://www.nuget.org/api/v2/" />
   </packageSources>
   <disabledPackageSources />