Oh right, we intentionally fail if there's no SECRET_TOKEN provided, but
that's not really useful for development!
Here, we add a SECRET_TOKEN only used in development - which doesn't
need to be secret, because it doesn't guard actual user sessions!
In production, the behavior is unchanged.
NOTE: This doesn't boot yet! There's something changed in the `devise` API that we'll need to fix!
```
/vagrant/config/initializers/devise.rb:46:in `block in <top (required)>': undefined method `encryptor=' for Devise:Module (NoMethodError)
```
But yeah, we navigated the gem upgrades, and also I ran `rake rails:update` and hand-processed the suggestions it had for our config files.
