Fix PHP version requirement

README.md says

```
Requirements
This library package requires PHP 5.4 or later and Plesk 12.0 or above.
```

composer.json says

```
"require": {
        "php": ">=5.3.0"
    }
```

So which one is it, does it require PHP >=5.3 or >=5.4? Either way, the version numbers should be updated to match.