Safer secret token

More secure `secret_token`.
Rails4 now requires `secret_key_base` or `secret_token` to be defined, otherwise an error is raised.
Changed `secret_key_base` to avoid deprecation warning.

Ref: 
http://daniel.fone.net.nz/blog/2013/05/20/a-better-way-to-manage-the-rails-secret-token/