For deploying any application CMS/FW based on PHP, you need to add composer.json at root as heroku default will run composer install.
composer.json
So just add composer.json without any dependency at root ( same level as of wp-config ) then it will work perfectly
There is a special repository for WordPress on Heroku.
https://github.com/php4dev/heroku-wordpress
This github repository contains composer.json for Wordpress.