add sonata admin bundle as a requirement

composer install didn't work with current composer.json

Was erroring with:

PHP Fatal error:  Class 'Sonata\AdminBundle\Admin\Admin' not found in /Users/danielplatt/Sites/myphone/vendor/liuggio/help-desk-bundle/Liuggio/HelpDeskBundle/Admin/TicketStateAdmin.php on line 11

Update includes sonata admin bundle