fix: use promisify-es6 instead of Nodes.js' promisify

js-ipfs is using the promisify-es6 module, so it makes sense that
we use it as well.

Author: vmx

package.json

@@ -62,6 +62,7 @@
     "ipld-raw": "^2.0.1",
     "merge-options": "^1.0.1",
     "multicodec": "~0.5.0",
+    "promisify-es6": "^1.0.3",
     "typical": "^3.0.0"
   },
   "contributors": [

src/index.js

@@ -1,14 +1,13 @@
 'use strict'
 
-const promisify = require('util').promisify
-
 const Block = require('ipfs-block')
 const CID = require('cids')
 const mergeOptions = require('merge-options')
 const ipldDagCbor = require('ipld-dag-cbor')
 const ipldDagPb = require('ipld-dag-pb')
 const ipldRaw = require('ipld-raw')
 const multicodec = require('multicodec')
+const promisify = require('promisify-es6')
 const typical = require('typical')
 const { extendIterator } = require('./util')