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GitHub User: colszowka
You should be careful with that bundle update - if you run it, not only will rails and its dependencies be updated, but also everything else. If you are not careful with the version numbers you specifiy in your Gemfile, you might end up with potentially breaking updates of the gems you're depending upon. I think for updating Rails it's better to first bundle update rails. If everything works, go on and update all your other gems.
bundle update rails