Trivial Description
According to https://pages.github.com/versions/, the current version of the github-pages Gem is 69 (I presently have 43 in my Gemfile.lock).
To make sure we're all building consistent versions of the website, we should specify a version of the github-pages gem (which will also ensure that we have consistent versions of Jekyll in use).
We'll probably have to make a glance over the website to make sure everything is still rendered correctly after the version update.
If that goes well, we should remove the explicit jekyll gem dependency in Gemfile and just add a specific github-pages version instead.