Fix which node version is required

In `package.json` engine is set to `>=9.0.0`.

README.md

@@ -86,7 +86,7 @@ The core of reveal.js is very easy to install. You'll simply need to download a
 
 Some reveal.js features, like external Markdown and speaker notes, require that presentations run from a local web server. The following instructions will set up such a server as well as all of the development tasks needed to make edits to the reveal.js source code.
 
-1. Install [Node.js](http://nodejs.org/) (4.0.0 or later)
+1. Install [Node.js](http://nodejs.org/) (9.0.0 or later)
 
 1. Clone the reveal.js repository
    ```sh