fix factoryFor error

Fix of the error below : 

You attempted to access "ifa" on a factory manager created by container#factoryFor. "ifa" is not a member of a factory manager."

addon/initializers/asset-map.js

@@ -7,7 +7,7 @@ export function initialize(app) {
   const container = app.__container__;
   let config;
   if(container.factoryFor) {
-    config = container.factoryFor('config:environment');
+    config = container.factoryFor('config:environment').class;
   } else {
     config = container.lookupFactory('config:environment');
   }