I got this error in codeship, when it trying to push the project into heroku:
Warning: Permanently added 'heroku.com,184.108.40.206' (RSA) to the list of known hosts. Permission denied (publickey). fatal: Could not read from remote repository.
It usually have no problem.
My solution is :
in codeship, open current project's general project settings,
copy the content which covered by red lines:
past what you copied into the input box and it will automatically be recognized as a ssh key.