composer php version does not match code syntax

Hi,

The composer.json say `"php": ">=7.1"` but the code use "|" type operator syntax (`string|array`) that is not compatible with Php 7

```
public function execute(string|array $command): SshCommand
```

Upgrading the composer Php requirement or downgrading the Php syntax ?

_I need your minimal Php version choice before proposing some Pull requests_ ;-)