title: Class constructors cannot be invoked without 'new'
body: <p>Just upgraded to node 4.1.2 and using Mongorito (which uses ES6) to access Mongo and I'm getting this:</p>
  
  <p>Model file:</p>
  
  <pre><code>var Mongorito = require('mongorito');
  var Model = Mongorito.Model;
  var config = require('../config/config');
  Mongorito.connect(config.mongo.url);
  
  class Listing extends Model {}
  
  module.exports = Listing;
  </code></pre>
  
  <p>And I'm including it:</p>
  
  <pre><code>var Listing = require('../models/listing');
  var listing = yield Listing.where('cacheKey', key).findOne();
  </code></pre>
  
  <pre class="lang-none prettyprint-override"><code>TypeError: Class constructors cannot be invoked without 'new'
        at Listing.Model (/node_modules/mongorito/lib/mongorito.js:140:15)
        at new Listing (/models/listing.js:7:14)
        at Query.find (/node_modules/mongorito/lib/query.js:355:21)
        at [object Generator].next (native)
        at onFulfilled (/node_modules/koa/node_modules/co/index.js:65:19)
        at run (/node_modules/babel/node_modules/babel-core/node_modules/core-js/modules/es6.promise.js:89:39)
        at /node_modules/babel/node_modules/babel-core/node_modules/core-js/modules/es6.promise.js:100:28
        at flush (/node_modules/babel/node_modules/babel-core/node_modules/core-js/modules/$.microtask.js:17:13)
        at doNTCallback0 (node.js:408:9)
        at process._tickCallback (node.js:337:13)
  </code></pre>