Use 'stream' instead of 'readable-stream'

This decreases browser bundle size by nearly 100KB.

Before:
browserify -r bncode | wc -c
  246124

After:
browserify -r bncode | wc -c
  151471
update travis.yml
Merge pull request #13 from feross/smaller-bundle

Use 'stream' instead of 'readable-stream'
Remove bops. browserify's buffer works better now!

Browserify now uses native-buffer-browserify
(https://github.com/feross/native-buffer-browserify), a browser Buffer
implementation that is backed by Uint8Array so it’s fast. And we get
to keep the node API that we’re familiar with. Best of both worlds!