Use a new class instead of a deprecated one

Hi!

When I used vagrant-aws with the latest vagrant release v1.6.3, the following message was displayed.

```
Bringing machine 'default' up with 'aws' provider...
==> default: HandleBoxUrl middleware is deprecated. Use HandleBox instead.
==> default: This is a bug with the provider. Please contact the creator
==> default: of the provider you use to fix this.
```

I've replaced the deprecated class (HandleBoxUrl) with the new one.