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'