bump browserify to v6

Streams are broken in 5.5.0 for some reason. Let’s use v6.
2.1.0
Bump chokidar to ^0.10.2
Clarify that this incrementally rebuilds

I was unaware that this module did incremental builds until Max mentioned it on twitter: https://twitter.com/maxogden/status/451609829648244736 This should be more prominent in the README
set max listeners on event emitter instances. Fixes maxListeners
warnings when watching multiple bundles.
2.1.1