Add a self-update command to the `graph-composer.phar`. Possibly related: - [fabpot/PHP-CS-Fixer](https://github.com/fabpot/PHP-CS-Fixer/blob/master/Symfony/CS/Console/Command/SelfUpdateCommand.php) - [composer/composer](https://github.com/composer/composer/blob/master/src/Composer/Command/SelfUpdateCommand.php) - [herrera-io/php-phar-update](https://github.com/herrera-io/php-phar-update)