Update composer.json to use this branch as 5.8-dev
Update composer.json to use this branch as 5.8-dev

giggsey/libphonenumber-for-php,BenMorel/libphonenumber-for-php,sudiptpa/libphonenumber-for-php,Xephi/libphonenumber-for-php,gencer/libphonenumber-for-php,digideskio/libphonenumber-for-php,lukebeer/libphonenumber-for-php