Demo app can't run on Ruby 2.6.0 about demo-rails-app HOT 5 CLOSED

$ bundle update
Fetching gem metadata from http://rubygems.org/.............
Fetching gem metadata from http://rubygems.org/..
Resolving dependencies...
Bundler could not find compatible versions for gem "bundler":
  In Gemfile:
    rails (= 4.2.0) was resolved to 4.2.0, which depends on
      bundler (< 2.0, >= 1.3.0)

  Current Bundler version:
    bundler (2.0.1)
This Gemfile requires a different version of Bundler.
Perhaps you need to update Bundler by running `gem install bundler`?

Could not find gem 'bundler (< 2.0, >= 1.3.0)', which is required by gem 'rails (= 4.2.0)', in any of the sources.

$ sudo gem uninstall bundler
$ sudo gem install bundler --version '1.17.3'