Heroku 'Could not detect rake tasks' - ruby-on-rails-3

Question

I have a legacy app deployed to Heroku that is suddenly failing to compile. It works fine locally, and had no problems when I last deployed it on 3/23. Now I'm trying to push a small change - a single link added to a view.
I know this app is using unsupported versions of Ruby / Rails / Spreecommerce - 1.9.3 / 3.0.20 / 0.60. We have plans to update, but in the meantime I need to get this change live.
The error from Heroku is:
remote: ! Could not detect rake tasks
remote: ! ensure you can run `$ bundle exec rake -P` against your app
remote: ! and using the production group of your Gemfile.
That rake task completes locally. I double checked the Rakefile, but don't see any issues there. It also been updated in 4 years and hasn't been a problem for multiple deploys per month in the meantime. There also haven't been any Gemfile changes in 6 months and I've confirmed the app runs locally.
Here's my Gemfile (with an obfuscated personal repo of a previous dev):
1 source 'http://rubygems.org'
2
3 ruby "1.9.3"
4
5 gem 'rails', '3.0.20'
6
7 # Generic gem dependencies first
8 gem 'rake', '0.8.7'
9 gem 'aws-s3', :require => 'aws/s3'
10 gem 'dynamic_form'
11 gem 'heroku'
12 gem 'memcache-client'
13 gem 'net-sftp', '~> 2.0.5'
14 gem 'net-ssh', '~> 2.0.9'
15 gem 'fastercsv'
16
17 # Followed by spree itself first, all spree-specific extensions second
18 gem 'savon', '0.9.7'
19 gem 'spree', '0.60.4', :git => 'git://github.com/[some_dev_personal_repo]/spree.git'
20 gem 'spree_heroku', '1.0.0', :git => 'git://github.com/paxer/spree-heroku.git'
21 gem 'spree_advanced_cart', :git => 'git://github.com/romul/spree_advanced_cart.git', :branch => '0-60-x'
22
23 gem 'rack-timeout'
24
25 # Dev/Test/Stage/Prod gems
26 group :production, :staging do
27 gem 'pg'
28 #gem 'rails_12factor'
29 end
30
31 group :development, :test do
32 gem 'sqlite3'
33 gem 'webrat'
34 gem 'cucumber-rails'
35 gem 'rspec-rails'
36 end
And here's my Rakefile (minus identifying namespace):
1 # Add your own tasks in files placed in lib/tasks ending in .rake,
2 # for example lib/tasks/capistrano.rake, and they will automatically be available to Rake.
3
4 require File.expand_path('../config/application', __FILE__)
5 require 'rake'
6
7 [namespace]::Application.load_tasks

Answer 1

Schneems at Heroku got to the bottom of this. Although there'd never been a version of the rack-timeout version of this app specified, that was the root of the problem.
I believe at some point as well the Gemfile.lock file was generated on a Windows machine, which was keeping Heroku couldn't reuse my lock file (this predates me - I've only ever deployed using OS X).
I specified gem 'rack-timeout', '0.3.2', ran bundle install, and lo and behold the deploy was successful.
Many thanks to Schneems!