Add support for broccoli environment variable

Also updates dependencies to newest versions, otherwise this breaks with newest version of broccoli because `broccoli.helpers.loadBrocfile()` is now `broccoli.loadBrocfile()`.

The environment variable is useful to have different builds for production and development, like it is the case in the [Broccoli Sample App](https://github.com/joliss/broccoli-sample-app).