use node: prefix for requires

mostly because code is being autogenerated by all the AI stuff using
this prefix. it's also used in the stack trace.

migrations/20170414193114-database-utf8mb4.js

@@ -1,6 +1,6 @@
 'use strict';
 
-var url = require('url');
+var url = require('node:url');
 
 exports.up = function(db, callback) {
     var dbName = url.parse(process.env.DATABASE_URL).path.substr(1); // remove slash