"Internal error" when pushing to Heroku - ruby

Question

Whenever I try to push this repo to Heroku, I get this absurdly unhelpful error:
Counting objects: 214, done.
Delta compression using up to 8 threads.
Compressing objects: 100% (204/204), done.
Writing objects: 100% (214/214), 196.99 KiB | 306 KiB/s, done.
Total 214 (delta 98), reused 0 (delta 0)
-----> Removing .DS_Store files
-----> Ruby/Rack app detected
-----> Using Ruby version: ruby-2.0.0
-----> Installing dependencies using Bundler version 1.3.2
Running: bundle install --without development:test --path vendor/bundle --binstubs vendor/bundle/bin --deployment
The source :rubygems is deprecated because HTTP requests are insecure.
Please change your source to 'https://rubygems.org' if possible, or 'http://rubygems.org' if not.
Fetching gem metadata from http://rubygems.org/.........
Fetching gem metadata from http://rubygems.org/..
Fetching git://github.com/datamapper/dm-core.git
Fetching git://github.com/datamapper/dm-aggregates.git
Fetching git://github.com/datamapper/dm-do-adapter
Fetching git://github.com/datamapper/dm-migrations.git
Fetching git://github.com/datamapper/dm-postgres-adapter.git
Fetching git://github.com/datamapper/dm-serializer.git
Fetching git://github.com/datamapper/dm-timestamps.git
Fetching git://github.com/datamapper/dm-transactions.git
Fetching git://github.com/datamapper/dm-types.git
Fetching git://github.com/datamapper/dm-validations.git
Installing addressable (2.2.8)
Installing backports (3.3.3)
Installing bcrypt-ruby (3.0.1)
Installing daemons (1.1.9)
Installing data_objects (0.10.13)
Installing descendants_tracker (0.0.1)
Installing virtus (0.5.5)
Using dm-core (1.3.0.beta) from git://github.com/datamapper/dm-core.git (at master)
Using dm-aggregates (1.3.0.beta) from git://github.com/datamapper/dm-aggregates.git (at master)
Using dm-do-adapter (1.3.0.beta) from git://github.com/datamapper/dm-do-adapter (at master)
Using dm-migrations (1.3.0.beta) from git://github.com/datamapper/dm-migrations.git (at master)
Installing do_postgres (0.10.13)
Using dm-postgres-adapter (1.3.0.beta) from git://github.com/datamapper/dm-postgres-adapter.git (at master)
Installing fastercsv (1.5.5)
Installing multi_json (1.3.7)
Using dm-serializer (1.3.0.beta) from git://github.com/datamapper/dm-serializer.git (at master)
Using dm-timestamps (1.3.0.beta) from git://github.com/datamapper/dm-timestamps.git (at master)
Using dm-transactions (1.3.0.beta) from git://github.com/datamapper/dm-transactions.git (at master)
Installing stringex (1.5.1)
Installing uuidtools (2.1.4)
Using dm-types (1.3.0.beta) from git://github.com/datamapper/dm-types.git (at master)
Using dm-validations (1.3.0.beta) from git://github.com/datamapper/dm-validations.git (at master)
Installing eventmachine (1.0.3)
Installing pg (0.16.0)
Installing rack (1.5.2)
Installing rack-protection (1.5.0)
Installing tilt (1.4.1)
Installing sinatra (1.4.3)
Installing thin (1.5.1)
Using bundler (1.3.2)
Your bundle is complete! It was installed into ./vendor/bundle
Cleaning up the bundler cache.
The source :rubygems is deprecated because HTTP requests are insecure.
Please change your source to 'https://rubygems.org' if possible, or 'http://rubygems.org' if not.
-----> WARNINGS:
You have not declared a Ruby version in your Gemfile.
To set your Ruby version add this line to your Gemfile:
ruby '2.0.0'
# See https://devcenter.heroku.com/articles/ruby-versions for more information."
-----> Discovering process types
Procfile declares types -> web
Default types for Ruby/Rack -> console, rake
-----> Compiled slug size: 44.4MB
-----> Launching...
! Push rejected, Internal error, please try again
To git#heroku.com:[...].git
! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 'git#heroku.com:[...].git'
I've tried deleting and recreating the app, deleting and recreating the Gemfile lock, adding a Procfile, and declaring Ruby version '1.9.3' (which I use locally). The app runs without complaint locally. What could be wrong?

Answer 1

Pushed the exact same code without modification to Heroku successfully an hour or so later.
This was probably a service problem with Heroku.