Fixed issue with incompatible buffer.toJSON for node 0.10 and 0.12

lib/encoding/bufferreader.js

@@ -91,8 +91,9 @@ BufferReader.prototype.readUInt64BEBN = function() {
 };
 
 BufferReader.prototype.readUInt64LEBN = function() {
-  var data = this.buf.slice(this.pos, this.pos + 8).toJSON().data;
-  var bn = new BN(data, 10, 'le');
+  var buf = this.buf.slice(this.pos, this.pos + 8);
+  var array = Array.prototype.slice.call(buf, 0);
+  var bn = new BN(array, 10, 'le');
   this.pos = this.pos + 8;
   return bn;
 };