Неопределенный метод auto_upgrade! при отправке приложения Sinatra / DataMapper в Heroku

Кто-нибудь знает магическое заклинание, необходимое для запуска приложения Sinatra, использующего DataMapper, на стеке Bamboo от Heroku? В стеке Bamboo нет предустановленных системных драгоценных камней, и независимо от того, какую комбинацию драгоценных камней я пробую, я получаю эту ошибку:

undefined method `auto_upgrade!' for DataMapper:Module (NoMethodError)

Это то, что у меня есть в моем файле .gems :

sinatra
pg
datamapper
do_postgres
dm-postgres-adapter

И вот зависимости, которые устанавливаются, когда я отправляю приложение на Heroku:

-----> Heroku receiving push
-----> Sinatra app detected

-----> Installing gem sinatra from http://rubygems.org
       Successfully installed sinatra-1.0
       1 gem installed

-----> Installing gem pg from http://rubygems.org
       Building native extensions.  This could take a while...
       Successfully installed pg-0.9.0
       1 gem installed

-----> Installing gem datamapper from http://rubygems.org
       Successfully installed extlib-0.9.15
       Successfully installed addressable-2.2.1
       Successfully installed dm-core-1.0.2
       Successfully installed dm-aggregates-1.0.2
       Successfully installed dm-migrations-1.0.2
       Successfully installed dm-constraints-1.0.2
       Successfully installed dm-transactions-1.0.2
       Successfully installed fastercsv-1.5.3
       Successfully installed json_pure-1.4.6
       Successfully installed dm-serializer-1.0.2
       Successfully installed dm-timestamps-1.0.2
       Successfully installed dm-validations-1.0.2
       Successfully installed uuidtools-2.1.1
       Successfully installed stringex-1.1.0
       Successfully installed dm-types-1.0.2
       Successfully installed datamapper-1.0.2
       16 gems installed

-----> Installing gem do_postgres from http://rubygems.org
       Building native extensions.  This could take a while...
       Successfully installed data_objects-0.10.2
       Successfully installed do_postgres-0.10.2
       2 gems installed

-----> Installing gem dm-postgres-adapter from http://rubygems.org
       Successfully installed dm-do-adapter-1.0.2
       Successfully installed dm-postgres-adapter-1.0.2
       2 gems installed

Приложение отлично работает локально с использованием SQLite3. Мне не хватает другого компонента PostgreSQL?