Gemfile
Where the Ruby version and gem dependencies are specified
The recommended content of your Gemfile is:
New Jekyll Theme (as of Winter 2019)
source 'https://rubygems.org'
ruby "2.5.1"
gem 'github-pages'
gem "jekyll-remote-theme"
gem "jekyll-include-cache"
Legacy repos (Before Winter 2019)
source 'https://rubygems.org'
ruby "2.5.1"
gem 'github-pages'