use object-assign instead of object.assign (close #1)

Author: eush77

index.js

@@ -1,6 +1,6 @@
 'use strict';
 
-var assign = require('object.assign');
+var assign = require('object-assign');
 
 
 module.exports = function u (type, props, value) {

package.json

@@ -28,7 +28,7 @@
     "unist"
   ],
   "dependencies": {
-    "object.assign": "^4.0.1"
+    "object-assign": "^4.1.0"
   },
   "devDependencies": {
     "tape": "^4.2.0"