Exclude vertx module from browser bundles

TL;DR: this fixes using this module in webpack.

I ran into an error while trying to use this module with [webpack](http://webpack.github.io). Apparently browserify does not recognize the [special require](https://github.com/conradz/when/blob/master/lib/timer.js#L13) that is used to test for vertx support, so it does not attempt to include it in the bundle. Webpack, however, does recognize that syntax and tries to find the `vertx` module, which of course fails. The [proper way](https://github.com/defunctzombie/node-browser-resolve#skip) to exclude a module from being included when targeting the browser is to specify `false` for the module in the `browser` field in the `package.json` file.