doc: update prequisites to node.js v6

Author: etpinard

CONTRIBUTING.md

@@ -32,7 +32,7 @@ We use the following [labels](https://github.com/plotly/plotly.js/labels) to tra
 #### Perequisites
 
 - git
-- [node.js](https://nodejs.org/en/). We recommend using node.js 4.2.x (LTS).
+- [node.js](https://nodejs.org/en/). We recommend using node.js v6.x or v4.x (both LTS).
   Upgrading and managing node versions can be easily done using
   [`nvm`](https://github.com/creationix/nvm) or its Windows alternatives.