bundle update nokogiri rack

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I managed to get the project up-and-running, then upgraded the gems we see GH security alerts for.

Now I'm stuck with

Liquid Exception: Liquid syntax error (line 76): Unknown tag 'gallery' in /mnt/shared/development/private/elixir.ruhr/_meetups/001-where-to-start.html

Should we convert the tags?

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Also, I cannot get the build on Travis to pick a recent ruby version. Neither setting the rvm key in .travis.yml nor adding a .ruby-version works.