Replace util._extend with Object.assign

This was never part of the public API, and has officially been
depreciated in Node 6 nodejs/node#4903

lib/index.js

@@ -3,7 +3,6 @@
  */
 
 var path = require('path'),
-  util = require('util'),
   clonedeep = require('lodash.clonedeep'),
   errors = require('./errors'),
   sass = require('./extensions');
@@ -289,7 +288,7 @@ module.exports.render = function(opts, cb) {
 
   // options.error and options.success are for libsass binding
   options.error = function(err) {
-    var payload = util._extend(new Error(), JSON.parse(err));
+    var payload = Object.assign(new Error(), JSON.parse(err));
 
     if (cb) {
       options.context.callback.call(options.context, payload, null);
@@ -437,7 +436,7 @@ module.exports.renderSync = function(opts) {
     return result;
   }
 
-  throw util._extend(new Error(), JSON.parse(result.error));
+  throw Object.assign(new Error(), JSON.parse(result.error));
 };
 
 /**