Use secrets.yml and .env instead of secret_token.rb

Does this look like a good way to set up secret_key_base? I followed advice from:

http://guides.rubyonrails.org/upgrading_ruby_on_rails.html#config-secrets-yml 

https://github.com/bkeepers/dotenv-deployment#capistrano-3 

@marklatham, you still need to initialize secret_key_base and you can do this with just one ENV variable like:

FirstApp::Application.config.secret_key_base = ENV['SECRET_KEY_BASE']