Update package.json

The last version(2.4.1) is officially deprecated.

Author: Paul Berg

quickstarts/email-users/functions/package.json

@@ -4,6 +4,6 @@
   "dependencies": {
     "firebase-admin": "^4.1.1",
     "firebase-functions": "^0.5.1",
-    "nodemailer": "^2.4.1"
+    "nodemailer": "^4.0.1"
   }
 }