Add BootstrapperCore nuget feed.

nuget.config

@@ -2,6 +2,7 @@
 <configuration>
   <packageSources>
     <clear />
+    <add key="wixtoolset-bootstrappercore" value="https://ci.appveyor.com/nuget/wixtoolset-bootstrappercore" />
     <add key="wixtoolset-dutil" value="https://ci.appveyor.com/nuget/wixtoolset-dutil" />
     <add key="api.nuget.org" value="https://api.nuget.org/v3/index.json" protocolVersion="3" />
   </packageSources>