Try running the following to list your ssh keys currently in your key chain:
ssh-add -l
You should see something like this:
2048 3c:a8:e1:...:f7 /Users/.../.ssh/github_rsa (RSA)
If you don't, it means your key is not loaded. Do:
ssh-add ~/.ssh/github_rsa
Then it should work
git push heroku master