Adding the private key to .gitignore and exclude list in _config.yml causes the app to crash

Hi there,

Thank you very much for the plugin. I followed your recommendation and added the private key to `.gitignore` file and to the exclude list in `_config.yml` file. But when I deploy my site on Heroku, this causes an ArgumentError and the app crashes. Is there any way to avoid it?

Thank you very much in advance.
