Default bucket to latest for psite-load-backup

In most cases, the latest backup will be desired (at least for me). This defaults $bucket to latest when it is omitted.

```
drush psite-load-backup mysite dev
```

This could be applied to site-dl-backup, but it would require a change of order in the arguments. For me, psite-load-backup will be used a lot more than download, so its not that important.