Specify which railties we want to require

We are being explicit about which railties we want to require in our
application. This for be able to deploy our application succesfully.

config/application.rb

@@ -2,7 +2,17 @@
 
 require_relative 'boot'
 
-require 'rails/all'
+%w(
+  active_record/railtie
+  action_controller/railtie
+  action_view/railtie
+  action_mailer/railtie
+).each do |railtie|
+  begin # rubocop:disable Style/RedundantBegin
+    require railtie
+  rescue LoadError
+  end
+end
 
 Bundler.require(*Rails.groups)