Replace util._extend with lodash.assign

This was never part of the public API, and has officially been
depreciated in Node 6 nodejs/node#4903
Object.assign can't be used yet since it requires ES2015

lib/index.js

@@ -3,9 +3,9 @@
  */
 
 var path = require('path'),
-  util = require('util'),
   clonedeep = require('lodash.clonedeep'),
   errors = require('./errors'),
+  assign = require('lodash.assign'),
   sass = require('./extensions');
 
 if (!sass.hasBinary(sass.getBinaryPath())) {
@@ -289,7 +289,7 @@ 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 = assign(new Error(), JSON.parse(err));
 
     if (cb) {
       options.context.callback.call(options.context, payload, null);
@@ -437,7 +437,7 @@ module.exports.renderSync = function(opts) {
     return result;
   }
 
-  throw util._extend(new Error(), JSON.parse(result.error));
+  throw assign(new Error(), JSON.parse(result.error));
 };
 
 /**

package.json

@@ -59,6 +59,7 @@
     "get-stdin": "^4.0.1",
     "glob": "^7.0.3",
     "in-publish": "^2.0.0",
+    "lodash.assign": "^4.2.0",
     "lodash.clonedeep": "^4.3.2",
     "meow": "^3.7.0",
     "mkdirp": "^0.5.1",