Sometimes you want to use a gem on Heroku that is in a private repository on GitHub.
Using git over http you can authenticate to GitHub using basic authentication. However, we don't want to embed usernames and passwords in Gemfiles. Instead, we can use authentication tokens.
This method does not add your OAuth token to Gemfile.lock. It uses bundle config to store your credentials, and allows you to configure Heroku to use environment variables when deploying.