Use dotenv-rails for environment-specific configuration

:question: 

One common question at the hackday last week was "should I configure Supermarket in config/application.yml or in .env?" so I started digging into using only one configuration mechanism. If the approach feels good, I'll add a Trello card. If not, I'll add more detailed docs regarding the difference between what's in `config/application.yml` and what's in `.env`.

`dotenv-rails` allows environment-specific configuration with files named `.env.#{Rails.env}`. We ignore `.env.development` and `.env.test` so that local configuration changes are not tracked, and use `.env.production` in production.