auth.getMobileSession should use HTTPS

According to the [documentation](http://www.last.fm/api/show/auth.getMobileSession), the `auth.getMobileSession` method will not work over plain HTTP. HTTPS should be used instead.

The returned error object looks like this:

```javascript
{ error: 4,
  message: 'You must use HTTPS in order to use this method.',
  links: [] }
```