Add lib folder path to eager_load_paths

Because autoloading is disabled in production with Rails 5,
using autoload_paths will not load needed classes from
specified paths. The solution to this, is to ask Rails
to eager load classes.

https://sipsandbits.com/2017/02/14/upgrading-a-ruby-on-rails-application/

config/environments/production.rb

@@ -10,6 +10,11 @@ Rails.application.configure do
   # Rake tasks automatically ignore this option for performance.
   config.eager_load = true
 
+  # Because autoloading is disabled in production environments with Rails 5,
+  # using autoload_paths will not load needed classes from specified paths.
+  # The solution to this, is to ask Rails to eager load classes.
+  config.eager_load_paths << "#{config.root}/lib"
+
   # Full error reports are disabled and caching is turned on.
   config.consider_all_requests_local       = false
   config.action_controller.perform_caching = true