upgrade packages to support ES6 transforms

More recent versions of reactify (which depend on browserify and
watchify) support ES6 transforms via package.json. This makes it easy
to use ES6 features like arrow functions without much additional code.

I upgraded the versions references in both the todo and chat examples,
and modified the chat example to use ES6 and arrow functions as an
example.
You don't even have to pass `this` anymore, it should be automatically bound by the arrow function.