Use dotenv gem to store environment variables

Hey, I noticed that you added a conditional for setting the secret keys so people don’t have to set their own environment variables. The [dotenv](https://github.com/bkeepers/dotenv) gem is well-suited to this task, so here’s an update that stores the keys you created in `.env.development` and `.env.test` so developers don’t have to fiddle with them but the initialiser code to store the variables is more DRY.