Mention NodeJS 4.0 dependency in the README (prebid#1386)

* Mention NodeJS 4.0 dependency in the README

This caused me some trouble on a Debian Jessie machine (which ships with NodeJS 0.10).

* Require NodeJS => 4.0

* (re)move note about NodeJS

README.md

@@ -28,10 +28,12 @@ Working examples can be found in [the developer docs](http://prebid.org/dev-docs
     $ cd Prebid.js
     $ yarn install
 
-Prebid also supports the `yarn` npm client. This is an alternative to using `npm` for package management, though `npm` will continue to work as before.
+Prebid supports the `yarn` npm client. This is an alternative to using `npm` for package management, though `npm install` will continue to work as before.
 
 For more info, see [the Yarn documentation](https://yarnpkg.com).
 
+*Note:* You need to have `NodeJS` 4.x or greater installed.
+
 <a name="Build"></a>
 
 ## Build for Development
@@ -47,8 +49,6 @@ This runs some code quality checks, starts a web server at `http://localhost:999
 + `./build/dist/prebid.js` - Minified production code
 + `./prebid.js_<version>.zip` - Distributable zip archive
 
-*Note:* You need to have `node.js` 4.x or greater installed to be able to run the `gulp build` commands.
-
 ### Build Optimization
 
 The standard build output contains all the available modules from within the `modules` folder.

package.json

@@ -20,6 +20,9 @@
   "globalVarName": "pbjs",
   "author": "the prebid.js contributors",
   "license": "Apache-2.0",
+  "engines": {
+    "node": ">=4.0"
+  },
   "devDependencies": {
     "babel-core": "6.22.0",
     "babel-loader": "^7.1.1",