Revert removal of packageSourceMapping.

Author: AndyButland

NuGet.config

@@ -6,7 +6,6 @@
     <add key="Umbraco Prereleases" value="https://www.myget.org/F/umbracoprereleases/api/v3/index.json" />
     <add key="Umbraco Nightly" value="https://www.myget.org/F/umbraconightly/api/v3/index.json" />
   </packageSources>
-  <!--
   <packageSourceMapping>
     <packageSource key="nuget.org">
       <package pattern="*" />
@@ -20,5 +19,4 @@
       <package pattern="Umbraco.Deploy.*" />
     </packageSource>
   </packageSourceMapping>
-  -->
 </configuration>