Add Production App Insights Key

The production environment configuration did not have a property for the appInsights key.  It was causing the build to fail when you run npm run dist in VSTS.

Author: Stewartarmbrecht

web/src/signalr-web/SignalRMiddleware/EventApp/src/environments/environment.prod.ts

@@ -1,3 +1,6 @@
 export const environment = {
-  production: true
+  production: true,
+  appInsights: { 
+    instrumentationKey: "%INSTRUMENTATION_KEY%" 
+  }
 };