The following Gemfile will permit you to run jekyll locally
source 'https://rubygems.org'
gem 'github-pages', group: :jekyll_plugins
gem 'github-linguist'
gem 'rouge', '~>1.11.1'
Write it as Gemfile under the repository root.
$ export GEM_HOME=$HOME/.gem
$ export PATH=$PATH:$HOME/.gem/bin
$ bundler install
$ bundle exec jekyll serve