title: Ruby on Rails asset precompilation fails on Heroku
body: <p>I have this problem that when I try to push to Heroku the error I get is</p>
  
  <pre><code>/tmp/build_c6e60f96-b7c3-4985-b3e0-d97e3a8408a8/vendor/bundle/ruby/2.0.0/gems/railties-4.0.0/lib/rails/initializable.rb:30:in `instance_exec'
  /tmp/build_c6e60f96-b7c3-4985-b3e0-d97e3a8408a8/vendor/bundle/ruby/2.0.0/gems/railties-4.0.0/lib/rails/initializable.rb:30:in `run'
  /tmp/build_c6e60f96-b7c3-4985-b3e0-d97e3a8408a8/vendor/bundle/ruby/2.0.0/gems/railties-4.0.0/lib/rails/initializable.rb:55:in `block in run_initializers'
  /tmp/build_c6e60f96-b7c3-4985-b3e0-d97e3a8408a8/vendor/bundle/ruby/2.0.0/gems/railties-4.0.0/lib/rails/initializable.rb:54:in `run_initializers'
  /tmp/build_c6e60f96-b7c3-4985-b3e0-d97e3a8408a8/vendor/bundle/ruby/2.0.0/gems/railties-4.0.0/lib/rails/application.rb:215:in `initialize!'
  /tmp/build_c6e60f96-b7c3-4985-b3e0-d97e3a8408a8/vendor/bundle/ruby/2.0.0/gems/railties-4.0.0/lib/rails/railtie/configurable.rb:30:in `method_missing'
  /tmp/build_c6e60f96-b7c3-4985-b3e0-d97e3a8408a8/config/environment.rb:5:in `&lt;top (required)&gt;'
  /tmp/build_c6e60f96-b7c3-4985-b3e0-d97e3a8408a8/vendor/bundle/ruby/2.0.0/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:228:in `require'
  /tmp/build_c6e60f96-b7c3-4985-b3e0-d97e3a8408a8/vendor/bundle/ruby/2.0.0/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:228:in `block in require'
  /tmp/build_c6e60f96-b7c3-4985-b3e0-d97e3a8408a8/vendor/bundle/ruby/2.0.0/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:213:in `load_dependency'
  /tmp/build_c6e60f96-b7c3-4985-b3e0-d97e3a8408a8/vendor/bundle/ruby/2.0.0/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:228:in `require'
  /tmp/build_c6e60f96-b7c3-4985-b3e0-d97e3a8408a8/vendor/bundle/ruby/2.0.0/gems/railties-4.0.0/lib/rails/application.rb:189:in `require_environment!'
  /tmp/build_c6e60f96-b7c3-4985-b3e0-d97e3a8408a8/vendor/bundle/ruby/2.0.0/gems/railties-4.0.0/lib/rails/application.rb:249:in `block in run_tasks_blocks'
  /tmp/build_c6e60f96-b7c3-4985-b3e0-d97e3a8408a8/vendor/bundle/ruby/2.0.0/gems/sprockets-rails-2.0.1/lib/sprockets/rails/task.rb:54:in `block (2 levels) in define'
  Tasks: TOP =&gt; environment
  (See full trace by running task with --trace)
  !
  !     Precompiling assets failed.
  !
  
  !     Push rejected, failed to compile Ruby app
  </code></pre>