gistfile1.rb

require 'rack'
require 'rack/contrib/try_static'

use ::Rack::TryStatic, 
  :root => "build",    	# where middleman files are generated
  :urls => %w[/]        	# match all requests
  :try => ['.html', 'index.html', '/index.html'] # try these postfixes sequentially
# 404
run lambda { [404, {'Content-Type' => 'text/plain'}, [' File not found.']]}