title: Rake aborted, Error while trying to load the gem 'uglifier'
body: <p>I'm following 'Getting Started with Ruby on Heroku', but when attempting to run the demo app locally with </p>
  
  <pre><code>$ bundle exec rake db:create db:migrate
  </code></pre>
  
  <p>I get</p>
  
  <pre><code>sam@samoliver:~/ruby-getting-started$ bundle exec rake db:create db:migrate --trace
  rake aborted!
  Bundler::GemRequireError: There was an error while trying to load the gem 'uglifier'.
  /home/sam/.rvm/gems/ruby-2.2.3/gems/bundler-1.11.0/lib/bundler/runtime.rb:80:in `rescue in block (2 levels) in require'
  /home/sam/.rvm/gems/ruby-2.2.3/gems/bundler-1.11.0/lib/bundler/runtime.rb:76:in `block (2 levels) in require'
  /home/sam/.rvm/gems/ruby-2.2.3/gems/bundler-1.11.0/lib/bundler/runtime.rb:72:in `each'
  /home/sam/.rvm/gems/ruby-2.2.3/gems/bundler-1.11.0/lib/bundler/runtime.rb:72:in `block in require'
  /home/sam/.rvm/gems/ruby-2.2.3/gems/bundler-1.11.0/lib/bundler/runtime.rb:61:in `each'
  /home/sam/.rvm/gems/ruby-2.2.3/gems/bundler-1.11.0/lib/bundler/runtime.rb:61:in `require'
  /home/sam/.rvm/gems/ruby-2.2.3/gems/bundler-1.11.0/lib/bundler.rb:99:in `require'
  /home/sam/ruby-getting-started/config/application.rb:7:in `&lt;top (required)&gt;'
  /home/sam/ruby-getting-started/Rakefile:4:in `require'
  /home/sam/ruby-getting-started/Rakefile:4:in `&lt;top (required)&gt;'
  /home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/rake_module.rb:28:in `load'
  /home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/rake_module.rb:28:in `load_rakefile'
  /home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/application.rb:689:in `raw_load_rakefile'
  /home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/application.rb:94:in `block in load_rakefile'
  /home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/application.rb:176:in `standard_exception_handling'
  /home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/application.rb:93:in `load_rakefile'
  /home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/application.rb:77:in `block in run'
  /home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/application.rb:176:in `standard_exception_handling'
  /home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/application.rb:75:in `run'
  /home/sam/.rvm/rubies/ruby-2.2.3/bin/rake:33:in `&lt;main&gt;'
  ExecJS::RuntimeUnavailable: Could not find a JavaScript runtime. See https://github.com/sstephenson/execjs for a list of available runtimes.
  /home/sam/.rvm/gems/ruby-2.2.3/gems/execjs-2.2.2/lib/execjs/runtimes.rb:51:in `autodetect'
  /home/sam/.rvm/gems/ruby-2.2.3/gems/execjs-2.2.2/lib/execjs.rb:5:in `&lt;module:ExecJS&gt;'
  /home/sam/.rvm/gems/ruby-2.2.3/gems/execjs-2.2.2/lib/execjs.rb:4:in `&lt;top (required)&gt;'
  /home/sam/.rvm/gems/ruby-2.2.3/gems/uglifier-2.5.3/lib/uglifier.rb:3:in `require'
  /home/sam/.rvm/gems/ruby-2.2.3/gems/uglifier-2.5.3/lib/uglifier.rb:3:in `&lt;top (required)&gt;'
  /home/sam/.rvm/gems/ruby-2.2.3/gems/bundler-1.11.0/lib/bundler/runtime.rb:77:in `require'
  /home/sam/.rvm/gems/ruby-2.2.3/gems/bundler-1.11.0/lib/bundler/runtime.rb:77:in `block (2 levels) in require'
  /home/sam/.rvm/gems/ruby-2.2.3/gems/bundler-1.11.0/lib/bundler/runtime.rb:72:in `each'
  /home/sam/.rvm/gems/ruby-2.2.3/gems/bundler-1.11.0/lib/bundler/runtime.rb:72:in `block in require'
  /home/sam/.rvm/gems/ruby-2.2.3/gems/bundler-1.11.0/lib/bundler/runtime.rb:61:in `each'
  /home/sam/.rvm/gems/ruby-2.2.3/gems/bundler-1.11.0/lib/bundler/runtime.rb:61:in `require'
  /home/sam/.rvm/gems/ruby-2.2.3/gems/bundler-1.11.0/lib/bundler.rb:99:in `require'
  /home/sam/ruby-getting-started/config/application.rb:7:in `&lt;top (required)&gt;'
  /home/sam/ruby-getting-started/Rakefile:4:in `require'
  /home/sam/ruby-getting-started/Rakefile:4:in `&lt;top (required)&gt;'
  /home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/rake_module.rb:28:in `load'
  /home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/rake_module.rb:28:in `load_rakefile'
  /home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/application.rb:689:in `raw_load_rakefile'
  /home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/application.rb:94:in `block in load_rakefile'
  /home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/application.rb:176:in `standard_exception_handling'
  /home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/application.rb:93:in `load_rakefile'
  /home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/application.rb:77:in `block in run'
  /home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/application.rb:176:in `standard_exception_handling'
  /home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/application.rb:75:in `run'
  /home/sam/.rvm/rubies/ruby-2.2.3/bin/rake:33:in `&lt;main&gt;'
  sam@samoliver:~/ruby-getting-started$ 
  </code></pre>
  
  <p>I tried removing and adding 'uglifier' (2.5.3) again to no effect and I do have 'therubyracer'  installed, so I don't understand what is ExecJS complaining about.</p>
  
  <p>Gemfile</p>
  
  <pre><code>source 'https://rubygems.org'
  ruby '2.2.3'
  
  # Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
  gem 'rails', '4.2.3'
  # Use postgresql as the database for Active Record
  gem 'pg'
  gem 'rails_12factor', group: :production
  # Use SCSS for stylesheets
  gem 'sass-rails', '~&gt; 5.0.3'
  # Use Uglifier as compressor for JavaScript assets
  gem 'uglifier', '&gt;= 1.3.0'
  # Use CoffeeScript for .js.coffee assets and views
  gem 'coffee-rails', '~&gt; 4.0.0'
  # See https://github.com/sstephenson/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', '~&gt; 2.0'
  # bundle exec rake doc:rails generates the API under doc/api.
  gem 'sdoc', '~&gt; 0.4.0',          group: :doc
  
  # Spring speeds up development by keeping your application running in the background. Read more: https://github.com/rails/spring
  gem 'spring',        group: :development
  
  # Use ActiveModel has_secure_password
  # gem 'bcrypt', '~&gt; 3.1.7'
  
  gem 'puma'
  
  # Use Capistrano for deployment
  # gem 'capistrano-rails', group: :development
  
  # Use debugger
  # gem 'debugger', group: [:development, :test]
  </code></pre>