title: "mpromise (mongoose's default promise library) is deprecated" error when testing
body: <p>For a while I have been getting this error and I decided to fix it today but after an hour trying to fix it I can find the solution.</p>
  
  <p>When I test my mongoose User model this error/warning is generated:</p>
  
  <pre><code>Mongoose: mpromise (mongoose's default promise library) is deprecated, plug in your own promise library instead: http://mongoosejs.com/docs/promises.html
  </code></pre>
  
  <p>Here is my test:</p>
  
  <pre><code>  1 var assert = require('chai').assert;
    2 var mongoose = require('mongoose');
    3 var clearDB = require('mocha-mongoose')(require('../../config/database').uri, { skip: ['workouts'] });
    4 var database = require('../../config/database').connect;
    5
    6 var User = require('../../app/models/user');
    7 var user = new User({});
    8
    9 var req_body = {
   10   username: "garyvee",
   11   email: "gary@vaynermedia.com",
   12   password: "secret"
   13 };
   14
   15 describe('User', function() {
   16   beforeEach(function(done) {
   17     user.username = "johnsmith";
   18     user.email = "john@gmail.com";
   19     user.password = "secret";
   20     done();
   21   });
   22
   23   it('can be saved', function() {
   24     return user.save(function(err: any) {
   25       assert.isNull(err);
   26     })
   27   });
   28 });
  </code></pre>
  
  <p>I think it has something to do with the <code>.save</code> but I don't know how to fix it. Can someone help me and tell me how to fix it so that error/warning isn't shown.</p>