Change default gravatar url to V2EX

config-example.json

@@ -113,7 +113,7 @@
   "rabbitMQ": "amqp://localhost/",
   "email_jwt_secret": "@EMAIL_JWT_SECRET@",
   "google_analytics": "UA-XXXXXXXX-X",
-  "gravatar_url": "https://gravatar.loli.net/avatar",
+  "gravatar_url": "https://cdn.v2ex.com/gravatar",
   "cdnjs_url": "https://cdnjs.loli.net/ajax/libs",
   "self_cdn_url": "",
   "google_fonts": {