Remove DB warning on connect

lib/db.js

@@ -8,7 +8,7 @@ function initDb(dbUrl, callback){ // eslint-disable-line
         console.warn('Trying to init DB again!');
         return callback(null, _db);
     }
-    MongoClient.connect(dbUrl, { useNewUrlParser: true }, connected);
+    MongoClient.connect(dbUrl, { useNewUrlParser: true, useUnifiedTopology: true }, connected);
     function connected(err, client){
         if(err){
             return callback(err);