update json-buffer

dunno why, but

```bash
$ npm install mux-demux
mux-demux@3.7.5 node_modules/mux-demux
├── stream-serializer@1.1.1
├── xtend@1.0.3
├── duplex@1.0.0
├── through@2.3.4
├── stream-combiner@0.0.2 (duplexer@0.0.4)
├── msgpack-stream@0.0.5 (msgpack-browserify@0.0.0, bops@0.0.6)
└── json-buffer@2.0.3 (bops@0.0.6)
```

doesn't give me the latest json-buffer