Callbackify getHeads in _init

Author: s1na

src/index.js

@@ -86,7 +86,7 @@ module.exports = class Blockchain {
 
     async.waterfall([
       (cb) => self._numberToHash(new BN(0), cb),
-      getHeads
+      util.callbackify(getHeads.bind(this))
     ], (err) => {
       if (err) {
         // if genesis block doesn't exist, create one