require moment where necessary

src/controllers/user.js

@@ -10,6 +10,7 @@ var User = require('./../models/user').model;
 var ga = require('./../utils').ga;
 var Group = require('./../models/group').model;
 var Challenge = require('./../models/challenge').model;
+var moment = require('moment');
 var logging = require('./../logging');
 var acceptablePUTPaths;
 var api = module.exports;