Add bonus for Rails 5.1 Encrypted Secrets

[jmmastey]

There's a new security-related feature that a lot of devs won't be familiar with:

http://weblog.rubyonrails.org/2017/4/27/Rails-5-1-final/#encrypted-secrets

Right now initializers/key.rb has a not-entirely-insecure approach of just completely omitting a production key, but the current state of the art is usually ENV based and I'd like to expand on that in the tutorial.

[jasnow]

Reference I just found: http://www.rubyflow.com/p/1o0pv1-screencast-encrypted-credentials-in-rails-52