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.

src/Microsoft.PowerShell.Linux.Host/project.json

@@ -25,5 +25,5 @@
 	}
     },
 
-    "frameworks": { "dnxcore50": { } }
+    "frameworks": { "dnxcore50": { "imports": "portable-net45+win8" } }
 }