heroku push error: "Could not detect rake tasks"

I'm trying to deploy a basic app and it's not working. Using

git push heroku master 

I got this error message:

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.
remote:  !     rake aborted!
remote:  !     LoadError: cannot load such file -- /tmp/build_a1cfcabff50107f84830d908386197b2/config/application
remote:  !     /tmp/build_a1cfcabff50107f84830d908386197b2/Rakefile:4:in `require'
remote:  !     /tmp/build_a1cfcabff50107f84830d908386197b2/Rakefile:4:in `<top (required)>'
remote:  !     /tmp/build_a1cfcabff50107f84830d908386197b2/vendor/bundle/ruby/2.2.0/gems/rake-11.1.2/lib/rake/rake_module.rb:28:in `load'
remote:  !     /tmp/build_a1cfcabff50107f84830d908386197b2/vendor/bundle/ruby/2.2.0/gems/rake-11.1.2/lib/rake/rake_module.rb:28:in `load_rakefile'
remote:  !     /tmp/build_a1cfcabff50107f84830d908386197b2/vendor/bundle/ruby/2.2.0/gems/rake-11.1.2/lib/rake/application.rb:689:in `raw_load_rakefile'
remote:  !     /tmp/build_a1cfcabff50107f84830d908386197b2/vendor/bundle/ruby/2.2.0/gems/rake-11.1.2/lib/rake/application.rb:94:in `block in load_rakefile'
remote:  !     /tmp/build_a1cfcabff50107f84830d908386197b2/vendor/bundle/ruby/2.2.0/gems/rake-11.1.2/lib/rake/application.rb:176:in `standard_exception_handling'
remote:  !     /tmp/build_a1cfcabff50107f84830d908386197b2/vendor/bundle/ruby/2.2.0/gems/rake-11.1.2/lib/rake/application.rb:93:in `load_rakefile'
remote:  !     /tmp/build_a1cfcabff50107f84830d908386197b2/vendor/bundle/ruby/2.2.0/gems/rake-11.1.2/lib/rake/application.rb:77:in `block in run'
remote:  !     /tmp/build_a1cfcabff50107f84830d908386197b2/vendor/bundle/ruby/2.2.0/gems/rake-11.1.2/lib/rake/application.rb:176:in `standard_exception_handling'
remote:  !     /tmp/build_a1cfcabff50107f84830d908386197b2/vendor/bundle/ruby/2.2.0/gems/rake-11.1.2/lib/rake/application.rb:75:in `run'
remote:  !     /tmp/build_a1cfcabff50107f84830d908386197b2/vendor/bundle/ruby/2.2.0/gems/rake-11.1.2/bin/rake:33:in `<top (required)>'
remote:  !     vendor/bundle/bin/rake:16:in `load'
remote:  !     vendor/bundle/bin/rake:16:in `<main>'
remote:  !
remote: /app/tmp/buildpacks/ruby/lib/language_pack/helpers/rake_runner.rb:102:in `load_rake_tasks!': Could not detect rake tasks (LanguagePack::Helpers::RakeRunner::CannotLoadRakefileError)

bundle exec rake -P seems to be working fine.

There's nothing special in my Gemfile, I just added Bootstrap:

source 'https://rubygems.org'


# Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
gem 'rails', '4.2.5.1'

# Use SCSS for stylesheets
gem 'sass-rails', '~> 5.0'
# Use Uglifier as compressor for JavaScript assets
gem 'uglifier', '>= 1.3.0'
# Use CoffeeScript for .coffee assets and views
gem 'coffee-rails', '~> 4.1.0'
# See https://github.com/rails/execjs#readme for more supported runtimes
# gem 'therubyracer', platforms: :ruby

# Use jquery as the JavaScript library
gem 'jquery-rails'
# Turbolinks makes following links in your web application faster. Read more: https://github.com/rails/turbolinks
gem 'turbolinks'
# Build JSON APIs with ease. Read more: https://github.com/rails/jbuilder
gem 'jbuilder', '~> 2.0'
# bundle exec rake doc:rails generates the API under doc/api.
gem 'sdoc', '~> 0.4.0', group: :doc

gem 'bootstrap-sass' 
# Use ActiveModel has_secure_password
# gem 'bcrypt', '~> 3.1.7'

# Use Unicorn as the app server
# gem 'unicorn'

# Use Capistrano for deployment
# gem 'capistrano-rails', group: :development
#postgreeSQL for Heroku deployment


group :development, :test do
  # Call 'byebug' anywhere in the code to stop execution and get a debugger console
  gem 'byebug'
  gem 'sqlite3'
end

group :development do
  # Access an IRB console on exception pages or by using <%= console %> in views
  gem 'web-console', '~> 2.0'


  # Spring speeds up development by keeping your application running in the background. Read more: https://github.com/rails/spring
  gem 'spring'
end
group :production do
  gem 'pg'
  gem 'rails_12factor'
end
ruby "2.2.3"

Well, I don't see why it's happening, since I have another project that is working normally. I checked everything and it still gives me that message.