add minimum order value to spree orders

checkout_controller_decorator.rb

Spree::CheckoutController.class_eval do
  def ensure_checkout_allowed
    check = @order.checkout_allowed?
    unless check == true
      redirect_to spree.cart_path, :flash => {:error => I18n.t("checkout_allowed_errors.#{check}")}
    end
  end
end

order_decorator.rb

Spree::Order.class_eval do
  MINIMUM_ORDER_VALUE = (ENV['MINIMUM_ORDER_VALUE'] || 20).to_i
  def checkout_allowed?
    return :not_empty     unless line_items.count > 0
    return :minimum_value unless total >= MINIMUM_ORDER_VALUE
    true
  end
end