Git pushing to Heroku raises a Bundler error - ruby-on-rails

Question

I'm trying to get an old Heroku app running, failing so far. $ heroku stack returns
=== <app_name> Available Stacks
bamboo-ree-1.8.7
cedar
* bamboo-mri-1.9.2 (prepared, will migrate on next git push)
and when pushing, I get the following:
$ git push heroku master
Counting objects: 48, done.
Delta compression using up to 4 threads.
Compressing objects: 100% (31/31), done.
Writing objects: 100% (31/31), 4.00 KiB, done.
Total 31 (delta 18), reused 0 (delta 0)
-----> Ruby/Rails app detected
-----> Detected Rails is not set to serve static_assets
Installing rails3_serve_static_assets... done
-----> Configure Rails 3 to disable x-sendfile
Installing rails3_disable_x_sendfile... done
-----> Configure Rails to log to stdout
Installing rails_log_stdout... done
-----> Gemfile detected, running Bundler version 1.0.7
Unresolved dependencies detected; Installing...
/tmp/build_3cljt2m8e8r7e/Gemfile:2:in `evaluate': undefined method `ruby' for #<Bundler::Dsl:0x00000003221778> (NoMethodError)
from /usr/ruby1.9.2/lib/ruby/gems/1.9.1/gems/bundler-1.0.7/lib/bundler/dsl.rb:7:in `instance_eval'
from /usr/ruby1.9.2/lib/ruby/gems/1.9.1/gems/bundler-1.0.7/lib/bundler/dsl.rb:7:in `evaluate'
from /usr/ruby1.9.2/lib/ruby/gems/1.9.1/gems/bundler-1.0.7/lib/bundler/definition.rb:17:in `build'
from /usr/ruby1.9.2/lib/ruby/gems/1.9.1/gems/bundler-1.0.7/lib/bundler.rb:128:in `definition'
from /usr/ruby1.9.2/lib/ruby/gems/1.9.1/gems/bundler-1.0.7/lib/bundler/cli.rb:225:in `install'
from /usr/ruby1.9.2/lib/ruby/gems/1.9.1/gems/bundler-1.0.7/lib/bundler/vendor/thor/task.rb:22:in `run'
from /usr/ruby1.9.2/lib/ruby/gems/1.9.1/gems/bundler-1.0.7/lib/bundler/vendor/thor/invocation.rb:118:in `invoke_task'
from /usr/ruby1.9.2/lib/ruby/gems/1.9.1/gems/bundler-1.0.7/lib/bundler/vendor/thor.rb:246:in `dispatch'
from /usr/ruby1.9.2/lib/ruby/gems/1.9.1/gems/bundler-1.0.7/lib/bundler/vendor/thor/base.rb:389:in `start'
from /usr/ruby1.9.2/lib/ruby/gems/1.9.1/gems/bundler-1.0.7/bin/bundle:13:in `<top (required)>'
from /usr/ruby1.9.2/bin/bundle:19:in `load'
from /usr/ruby1.9.2/bin/bundle:19:in `<main>'
FAILED: http://devcenter.heroku.com/articles/bundler
! Heroku push rejected, failed to install gems via Bundler
To git#heroku.com:<app_name>.git
! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 'git#heroku.com:<app_name>.git'
Current Gemfile:
source 'https://rubygems.org'
ruby '1.9.3'
gem 'bundler', '1.3.0.pre.8'
gem 'rails', '~> 3.2'
gem 'jquery-rails', '~> 2.2'
gem 'slim', '~> 1.3'
gem 'dynamic_form', '~> 1.0'
gem 'state_machine', '~> 1.1'
gem 'draper', '~> 1.1'
gem 'bcrypt-ruby', '~> 3.0', :require => 'bcrypt'
gem 'thin', '~> 1.5'
gem 'twitter-bootstrap-rails', '~> 2.2'
group :assets do
gem 'sass-rails', '~> 3.2.3'
gem 'less-rails', '~> 2.2'
gem 'coffee-rails', '~> 3.2.1'
gem 'therubyracer', '~> 0.11', :platforms => :ruby
gem 'uglifier', '>= 1.0.3'
end
group :development do
gem 'sqlite3', '~> 1.3'
gem 'pry', '~> 0.9'
gem 'better_errors', '~> 0.3'
gem 'binding_of_caller', '~> 0.6'
end
group :production do
gem 'pg', '~> 0.14'
end
It seems there is an issue with Bundler, any ideas?

Answer 1

The problem is caused by the line 2 of Gemfile:
ruby '1.9.3'
From the output we can see the version of Bundler running:
Gemfile detected, running Bundler version 1.0.7
But ruby directive seems to be Bundler 1.2 feature and won't work with old Bundler. I think you can safely remove this ruby '1.9.3' line for now.
Of course, you have gem 'bundler', '1.3.0.pre.8' in your gemfile. But this is the chicken and egg problem: new Bundler will not be installed until Gemfile is parsed, but old Bundler cannot parse it.