require rails/generators for generators

Because this code resides in lib/ it may be eager loaded. Rails in turn
doesn't define Rails::Generators by default unless you explicitly
require "rails/generators".

lib/generators/rails/post_deployment_migration/post_deployment_migration_generator.rb

+require 'rails/generators'
+
 module Rails
   class PostDeploymentMigrationGenerator < Rails::Generators::NamedBase
     def create_migration_file