fix: 云函数独立运行时，增加 / 路由，用来通过云引擎健康检查

Author: sdjcw

lib/cloud.js

@@ -220,7 +220,11 @@ Cloud.start = function() {
       masterKey: process.env.LEANCLOUD_APP_MASTER_KEY,
     });
   }
-  server = AV.express().listen(PORT, function() {
+  const app = AV.express();
+  app.use('/', function(req, res) {
+    res.end('This is a LeanEngine application.');
+  });
+  server = app.listen(PORT, function() {
     console.log('LeanEngine Cloud Functions app is running, port:', PORT);
   });
 };