Updated Gemfile for compatibility with jekyll docker images

[BenBlumer]

I'm submitting this one humbly as someone brand new to Ruby.

I was getting tons of errors trying to build the site locally -- despite that it worked fine on Github pages. So I tried a number of Docker containers and was still getting miscellaneous errors.

With this patch, it will build using the JV-conseil/jekyll-docker image. It also continues to build on Github pages.