Use `ENV.fetch` instead of `ENV[]`

It's easy to forget to set environment variables (e.g. `AWS_SECRET_KEY`) on
staging or production. Using `ENV.fetch("AWS_SECRET_KEY")` ensures that missing
environment variables raise errors instead of just being `nil`.

Heroku boots the app on deploy, which runs the app's initializers. Often,
environment variables are set in an initializer. This combination means that if
an environment variable isn't set, an error will be raised and Heroku will abort
the deploy. This means users can never see a version of the app without the environment
variables set.