Build production environment but not development environment.

This includes:
* Run bundler with deployment mode
* Sepcify -E production when rackup.
I admit some ignorance of rackup's behavior, and I recognize that rackup treats "development" and "deployment" as special values (i.e. it sets certain default middlewares). However, does this set RACK_ENV to "deployment" rather than "production"? What's the standard practice here?