Update native-buffer-browserify to 2.0.0 for IE6-8 support

Summary of changes:
- Works in IE6-8 now! (for real this time!)
- Bundle size reduced by 40%! (down to 37KB from 60KB)
- Firefox performance significantly improved (many orders of magnitude) by removing Proxy implementation

More info: https://github.com/feross/native-buffer-browserify/issues/7