The use of config/app.rb makes working on Heroku a little more difficult due to the platform's readonly file system (everything that is to be deployed should be inside the Git repository directly).
I get around this using a production branch where app.rb is committed. This branch is pushed to Heroku but obviously never to GitHub.
git checkout -b production
cp config/app.rb.example config/app.rb
<edit app.rb>
git commit config/app.rb
When making a change to the Rails app, checkout master, commit the changes, then rebase the production branch to master.
git checkout master
<edits>
git commit
git checkout production
git rebase master
Deploy production to Heroku as described on Stack Overflow:
git push heroku production:master
It's often useful to have both a staging and production Heroku app, and if you do, it's very convenient to link a local branch to each of these remote applications. This process is described in Heroku's docs.