bump to v4.0.0

* dropping support for node < 4
* signal the CI not to test node < 4

Author: refack

package.json

@@ -11,7 +11,7 @@
     "bindings",
     "gyp"
   ],
-  "version": "3.6.2",
+  "version": "4.0.0",
   "installVersion": 9,
   "author": "Nathan Rajlich <[email protected]> (http://tootallnate.net)",
   "repository": {