TypeError: Cannot read property 'service' of undefined

index.js

var feathers = require('feathers');
var hooks = require('feathers-hooks');
var rest = require('feathers-rest');
var socketio = require('feathers-socketio');
var bodyParser = require ('body-parser');
var authentication = require ('feathers-authentication').default;
var authHooks = require('feathers-authentication').hooks;
var mongoose = require('mongoose');
var service = require ('feathers-mongoose');

mongoose.Promise = global.Promise;
mongoose.connect('mongodb://localhost:27017/riddle');

var app = feathers()
    .configure(rest())
    .configure(socketio())
    .configure(hooks())
    .use(bodyParser.json())
    .use(bodyParser.urlencoded({ extended: true }))
    .configure(authentication({
        token:{
            secret: 'InYATTDuYA'
        },
        local: {
            usernameField:"username"
        }
    }));

// Define Schema
var Schema = mongoose.Schema;
var userSchema = new Schema({
  username: {
      type: String,
      required: true,
      index: {unique: true, dropDups: true}
  },
  password: {type: String, required: true},
  createdAt: {type: Date, 'default': Date.now},
});
var userModel = mongoose.model('user', userSchema);

// route user
app.use('/', service({Model: userModel}));

// hash password hook
var userService =  app.service('/');
userService.before(
    {
        create: [authHooks.hashPassword('password')]
    }
)

// jsonify error
app.use(function(error, req, res, next){
    debugger;
    console.log(error)
    res.status(error.code);
    res.json(error);
  })

app.listen(3000);

package.json

{
  "name": "some-problem",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "body-parser": "^1.9.0",
    "feathers": "2.0.0-pre.4",
    "feathers-hooks": "^1.0.0-pre.4",
    "feathers-mongoose": "^3.3.0",
    "feathers-primus": "^1.3.2",
    "feathers-rest": "^1.2.2",
    "feathers-socketio": "^1.3.2",
    "mongoose": "^4.4.3",
    "socket.io-client": "^1.1.0"
  }
}

One thing, your devDependencies should be dependencies