api_constraints.rb

class APIConstraints

  def initialize(options)

		@version = options[:version]
		@default = options[:default]
	end

	def matches?(request)

		@default || request.headers["Accept"].include?("application/vnd.kunto.v#{@version}")
	end
end

routes.rb

require "api_constraints"

API::Application.routes.draw do

  scope module: :V1, constraints: APIConstraints.new(version: 1, default: true), defaults: { format: :json } do

		resources :products
	end
end