build(node): update node from 4.1.1 to 4.2.1

This is just a maintenance upgrade to keep us close to the latest release.

No known bugs are being fixed by this upgrade.

I also removed the npm override in .travis.yaml since node 4 ships with a recent version of npm
and usually this version is preferred (it might contain custom patches).

Closes #4939

gulpfile.js

@@ -47,8 +47,8 @@ var sauceConf = require('./sauce.conf');
 var os = require('os');
 
 require('./tools/check-environment')({
-  requiredNpmVersion: '>=2.14.5',
-  requiredNodeVersion: '>=4.1.1'
+  requiredNpmVersion: '>=2.14.7',
+  requiredNodeVersion: '>=4.2.1'
 });
 
 // Make it easy to quiet down portions of the build.