Merge pull request #4086 from AnalyticalGraphicsInc/doc-node-version

Document required version of node in Build Guide

Documentation/Contributors/BuildGuide/README.md

@@ -26,7 +26,7 @@
 ## Build the Code
 
 Prerequisites:
- * Install [Node.js](http://nodejs.org/) on your system.
+ * Install [Node.js](http://nodejs.org/) on your system.  Building Cesium requires Node 4.x or newer.
 
 Cesium uses npm modules for development, so after syncing, you need to run `npm install` from the Cesium root directory:
 
@@ -160,4 +160,4 @@ To configure the additional commit statuses on GitHub for your fork of Cesium, y
 * Copy the token to your clipboard
 * Go to [travis.org](https://travis-ci.org/) and select your fork of Cesium
 * Go to "More Options">"Settings"
-* Under the "Environment Variables" section, add the environment variable `TOKEN` and paste your token for the value
+* Under the "Environment Variables" section, add the environment variable `TOKEN` and paste your token for the value