Why am I getting an error on Heroku that suggests I need to migrate my app to Bamboo?

When I type: git push heroku master, this is what happens

$:sample_app git push heroku master
Counting objects: 110, done.
Delta compression using up to 2 threads.
Compressing objects: 100% (94/94), done.
Writing objects: 100% (110/110), 87.48 KiB, done.
Total 110 (delta 19), reused 0 (delta 0)

-----> Heroku receiving push
-----> Rails app detected
 !     This version of Rails is only supported on the Bamboo stack
 !     Please migrate your app to Bamboo and push again.
 !     See http://docs.heroku.com/bamboo for more information
 !     Heroku push rejected, incompatible Rails version

error: hooks/pre-receive exited with error code 1
To git@heroku.com:blazing-frost-89.git
 ! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 'git@heroku.com:blazing-frost-89.开发者_JS百科git'

My .gems file:

rails --version 2.3.8

My .git/config file:

[core]
  repositoryformatversion = 0
  filemode = true
  bare = false
  logallrefupdates = true
  ignorecase = true
[remote "origin"]
  url = git@github.com:csmeder/sample_app.git
  fetch = +refs/heads/*:refs/remotes/origin/*
[remote "heroku"]
  url = git@heroku.com:blazing-frost-89.git
  fetch = +refs/heads/*:refs/remotes/heroku/*

Like it says, Rails 2.3.8 is only available on their Bamboo stack. I think it's because Bamboo is slated to become the default stack in the near future, and Rails has only been upgraded on that stack.