Added secret key to devise.

There is a problem in devise 3.0.3 that throws an error when creating accounts (https://github.com/plataformatec/devise/issues/2574). This may be unique to scoped Models, but I'm not sure since we don't have a sign up process for CMS users. 

Anyway, I needed to update the version, which means I now need to add a secret_key to the devise config for devise to work (http://blog.plataformatec.com.br/2013/08/devise-3-1-now-with-more-secure-defaults/). Wanted to check with you on this before I committed any changes as this might affect others using the CMS.