nodejs is required

It is a requirement to have NodeJS installed to run jekyll.

http://jekyllrb.com/docs/installation/
> NodeJS, or another JavaScript runtime (for CoffeeScript support).

```
$ jekyll serve/usr/local/lib/ruby/gems/2.0.0/gems/execjs-2.2.2/lib/execjs/runtimes.rb:51:in `autodetect': Could not find a JavaScript runtime. See https://github.com/sstephenson/execjs for a list of available runtimes. (ExecJS::RuntimeUnavailable)
  from /usr/local/lib/ruby/gems/2.0.0/gems/execjs-2.2.2/lib/execjs.rb:5:in `<module:ExecJS>'
```

I don't know what is the best way to give a command to install nodejs, maybe just leave it like that