Package no longer compatible with php version < 7.1

`src/Client.php` on line 68:

```
public function __construct($appName, $appVersion, $machineName="", $environment="", $guzzleParams = [], ?LoggerInterface $logger = null, $logRequestAndResponseBody = false)
    {
```

`?LoggerInterface $logger` is breaking my php 5.6 application but in the composer file/on packagist is indicated this package should be compatible with php >=5.5.9.

Thank you.