Fix possible fatal error while getting public IP

This simple pull request fixes a possible fatal error that can occur if the http request made when trying to lookup the public IP fails.

The error I was receiving was
```
PHP Fatal error:  Cannot use object of type WP_Error as array in 
.../plugins/uploads-by-proxy/class-get-public-ip.php on line 62
```
The `$response` was coming back as a WP_Error `http_request_failed: couldn't connect to host at hostnametoip.com:80`.