Reduce required Node.js version to 4.0.0.

The development workflow can be used with Node 4+, not just Node 6. We
should avoid showing warnings and errors when users are installing this
package via npm/yarn.

Ref. #1729, #1743.
Closes #1725.

package.json

@@ -19,7 +19,7 @@
     "url": "git://github.com/hakimel/reveal.js.git"
   },
   "engines": {
-    "node": "~6.9.1"
+    "node": ">=4.0.0"
   },
   "dependencies": {
     "express": "~4.14.0",