You can't push straight from Github to Heroku.
You're going to have to use the third host to coordinate the push. This could be fired from a Github post-receive hook.
To sync straight across use something like:
git remote add github git@github.com:user/repo.git
git remote add heroku git@heroku.com:app.git
git push heroku refs/remotes/github/master:refs/heads/master
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