Revert "Merge pull request #714 from heroku/app-tiers"

This reverts commit 6078acb06a9f51befdcfad2177f48b8c6699e4e1, reversing
changes made to d42163589eb7cab0293aad55faaaf000b843c220.

Pull request #714 breaks the specs even with heroku-api 0.3.8 in the gemspec, so reverting for now.