return service worker from generate_asset_cache - fixes #90

lib/utils/generate_asset_cache.js

@@ -44,7 +44,9 @@ module.exports = function generate_asset_cache(clientInfo, serverInfo) {
 
 		server: {
 			entry: path.resolve(dest, 'server', serverInfo.assetsByChunkName.main)
-		}
+		},
+
+		service_worker
 	};
 };