Create Gemfile

https://help.github.com/articles/using-jekyll-with-pages/

... Jekyll - The main event. You'll want to create a file in your site's repository called Gemfile and add the line gem 'github-pages'. After that, simply run the command, bundle install and you're good to go. If you decided to skip step #2, you can still install Jekyll with the command gem install github-pages, but you may run into trouble down the line. Here’s an example of a Gemfile you can use (placed in the root directory of your repository): ...