doc: fix badge node > 6

Author: Unitech

README.md

@@ -13,8 +13,8 @@
    <img src="https://badge.fury.io/js/pm2.svg" alt="npm version" height="18">
 </a>
 
-<a href="https://img.shields.io/badge/node-%3E%3D4-brightgreen.svg" title="Node Limitation">
-   <img src="https://img.shields.io/badge/node-%3E%3D4-brightgreen.svg" alt="npm version" height="18">
+<a href="https://img.shields.io/badge/node-%3E%3D6-brightgreen.svg" title="Node Limitation">
+   <img src="https://img.shields.io/badge/node-%3E%3D6-brightgreen.svg" alt="npm version" height="18">
 </a>
 
 <a href="https://travis-ci.org/Unitech/pm2" title="PM2 Tests">