Update NAN to at least 2.3.2 (#1496)

This removes the deprecation warning in Node 6

Author: xzyfer

package.json

@@ -61,7 +61,7 @@
     "meow": "^3.7.0",
     "in-publish": "^2.0.0",
     "mkdirp": "^0.5.1",
-    "nan": "^2.2.0",
+    "nan": "^2.3.2",
     "node-gyp": "^3.3.1",
     "npmconf": "^2.1.2",
     "request": "^2.61.0",