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.
Thanks for the typo note. Yes, something like that will be evaluated at runtime (and so won't be caught immediately), but the ENV variables like AWS keys, Sendgrid keys, etc are usually set in config/initializers.