Incompatibility with WordPress 2.9.2

We say WP Remote is compatible with WordPress 2.9.2, but it's not because of this in our API:

```
case 'get_site_info' :

			$actions[$action] = array(
				'site_url'	=> get_site_url(),
				'home_url'	=> get_home_url(),
				'admin_url'	=> get_admin_url(),
				'backups'	=> function_exists( '_wprp_get_backups_info' ) ? _wprp_get_backups_info() : array()
			);
```

Each of those functions was introduced in 3.0.

Should we continue to support 2.9.2? If so, we'll need to come up with alternatives, and update Travis to run against 2.9.2