$ rails s /usr/local/rvm/gems/ruby-2.1.5/gems/execjs-2.6.0/lib/execjs/runtimes.rb:48:in `autodetect': Could not find a JavaScript runtime. See https://github.com/rails/execjs for a list of available runtimes. (ExecJS::RuntimeUnavailable) from /usr/local/rvm/gems/ruby-2.1.5/gems/execjs-2.6.0/lib/execjs.rb:5:in `<module:ExecJS>' from /usr/local/rvm/gems/ruby-2.1.5/gems/execjs-2.6.0/lib/execjs.rb:4:i