support rails 3 secret_token

[nijikon]

No description provided.

[nijikon]

/cc @pixeltrix

[pixeltrix]

@nijikon I think there's actually two scenarios we need to address

1. Rails 3.2.x
   Copy across app.secrets.secret_token
2. Rails 4.0.x
   Copy across app.secrets.secret_key_base and app.secrets.secret_token but only if they are set

This is to cover the various possible settings when upgrading from Rails 3.2.x to Rails 4.0.x. Additionally, I think we need to add a integration test that sets a value in a session and then goes to another page and asserts the value from the session.