Added concrete5 update

This allows to install concrete5 updates via Composer:

```
{
	"type" : "package",
	"package" : {
		"name" : "concrete5/concrete5",
		"version" : "5.6.3",
		"type" : "concrete5-update",
		"dist" : {
			"type" : "zip",
			"url" : "http://www.concrete5.org/download_file/-/view/65413/8497/"
		}
	}
}
```

The updated core can be activate in the Dashboard or by adding this line to `config/site.php`:

```
define('DIRNAME_APP_UPDATED', 'concrete5');
```

/cc @Remo