Use rackup rather than rails server in Procfile

[patcon]

If I submitted a PR for using bundle exec rackup config.ru $PORT, would y'all accept that? Assuming that's what's being used in production, so wondering if we might as well standardize on that.

[monfresh]

We don't use the Procfile at all in production. We use the Passenger nginx module to start the server via Capistrano, and we launch the Sidekiq workers via Capistrano as well, on a separate server, for example: https://github.com/18F/identity-idp/blob/master/config/deploy/dev.rb