Fix Date mocking regression introduced in 2.11.0

Fix Date mocking regression introduced in 2.11.0

The commit https://github.com/moment/moment/commit/f54a32f4c62c12a710335211a73cb002a7249646 and the follow on refactoring https://github.com/moment/moment/commit/b1d686ed481e2211cb81a1c60eca850fcea93f21 broke the ability for Date mocking libraries ([MockDate](https://www.npmjs.com/package/mockdate), SinonJS, etc.) to mock out the Date object. I believe we should restore this functionality.

```js
const moment = require('moment'),
    mockdate = require('mockdate'),
    frozenTime = moment();

mockdate.set(frozenTime);
console.log('Frozen time: ', frozenTime.toISOString());

setTimeout(function() {
    console.log('Moment time: ', moment().toISOString());
}, 1000);
```

Running the code above will yield the following in `moment@2.10.6` and `moment@2.11.0` respectively.

```bash
$ node index.js
Frozen time:  2016-01-04T14:21:55.335Z
Moment time:  2016-01-04T14:21:55.335Z

$ node index.js
Frozen time:  2016-01-04T14:21:55.335Z
Moment time:  2016-01-04T14:21:56.355Z
```

- Fixes #2857
- Fixes #2855