Fix missing YAML requirement

Trying to read `Rails.application.secrets.secret_key_base`
from a Rails::API application used to raise an error because
`Rails::Application::YAML` was undefined.

Aternatively, the issue can be worked around by requiring YAML
where the secrets will be read, but I think it is worth being fixed.

Example of workaround (config/initializers/secret_token.rb):

```
require 'yaml'
App.config.secret_key_base = Rails.application.secrets.secret_key_base
```