There was some drama around RubyGems.org (seems like a classic commercial overstep on a community resource).
And as a result there is now a new Gem server available: gem.coop

If you want to use the new server (which currently is a mirror of rubygems.org), simply replace source "https://rubygems.org" with source "https://gem.coop" in your Gemfile:

-source "https://rubygems.org"
+source "https://gem.coop"

I switched the Gemfile for this blog over to it, and so far it seems to work fine. 💎