$ bundle update
Fetching gem metadata from https://rubygems.org/............
Fetching gem metadata from https://rubygems.org/..
Resolving dependencies.......
Using concurrent-ruby 1.0.5
Using i18n 0.9.1
Using minitest 5.10.3
Using thread_safe 0.3.6
Using tzinfo 1.2.4
Using activesupport 4.2.9
Using public_suffix 2.0.5
Using addressable 2.5.2
Using bundler 1.16.1
Using coffee-script-source 1.11.1
Using execjs 2.7.0
Using coffee-script 2.4.1
Using colorator 1.1.0
Using ruby-enum 0.7.1
Using commonmarker 0.17.7.1
Fetching ffi 1.9.18 (x64-mingw32)
Installing ffi 1.9.18 (x64-mingw32)
Gem::RuntimeRequirementNotMetError: ffi requires Ruby version < 2.5, >= 2.0. The
current ruby version is 2.5.0.
An error occurred while installing ffi (1.9.18), and Bundler cannot continue.
Make sure that `gem install ffi -v '1.9.18'` succeeds before bundling.

In Gemfile:
  github-pages was resolved to 172, which depends on
    github-pages-health-check was resolved to 1.3.5, which depends on
      typhoeus was resolved to 0.8.0, which depends on
        ethon was resolved to 0.11.0, which depends on
          ffi
$ gem install ffi -v '1.9.18'
ERROR:  Error installing ffi:
        The last version of ffi (= 1.9.18) to support your Ruby & RubyGems was 1.9.18. Try installing it with `gem install ffi -v 1.9.18`
        ffi requires Ruby version < 2.5, >= 2.0. The current ruby version is 2.5.0.