Update gems

Security update for gems.

Hey @robguthrie, when you add gems in the future can you please make sure to add their versions to the Gemfile? I needed to update all the gems which used nokogiri, and normally I can just run `bundle update` and know that all of our gems will safely update their patch numbers. But because there were a few gems which didn't have any versions specified, I had to add those in before I could run `bundle update`.