swap order of subtract() args due to deprecation

From Moment.js docs [here](http://momentjs.com/docs/#/manipulating/subtract/):

Before version `2.8.0`, the `moment#subtract(String, Number)` syntax was also supported. It has been deprecated in favor of `moment#subtract(Number, String)`.

```javascript
moment().subtract('seconds', 1); // Deprecated in 2.8.0
moment().subtract(1, 'seconds');
```