Use bundler to run site

At the moment, the serve.sh script calls jekyll directly. However, this means that the jekyll version you are using might not be the same one that GitHub pages runs. To work around this, this pull request indicates through a Gemfile that the github-pages gem (aka bundle) should be installed. This will install the exact same versions of the gems that GitHub pages runs, which includes the correct Jekyll version.

Now all that is left is to automatically install the bundle in the serve.sh script and run the jekyll serve command through the bundle exec command, which will use the gems in the Gemfile (the GitHub pages one) to run Jekyll, thereby guaranteeing that you develop your website using a mirror of the GitHub pages website.

If GitHub pages release a new version of their Gem (which they did a couple of weeks ago), you can just type `bundle update github-pages`.