rails server したときに `autodetect': Could not find a JavaScript runtime. See https://github.com/sstephenson/execjs for a list of available runtimes. (ExecJS::RuntimeUnavailable)と出た時の対処法(といっても公式ドキュメントにそのまんま書いてあるけど・・・)。 Ruby on Rails Guides: Getting Started with Rails の 4.1 Starting up the Web Server に Compiling CoffeeScript to JavaScript requires a JavaScript runtime and the absence of a runtime will