Heroku will only deploy code pushed to the master branch of your heroku remote. Knowing this as the only method to push code to your remote is limiting if you want to deploy from a feature branch.
Luckily, Heroku does support pushing a non-master branch with simple change of syntax.
$ git push heroku feature/my-cool-feature:master
Simply tack on the :master and you can push your feature branch or any other branch for that matter.