Use rbenv? Need to update All The Rubies™ because of Bundler's BUNDLED_WITH change? Then use this script.

rbenv-update-bundler.sh

#!/bin/bash
eval "$(rbenv init -)"

CURRENT_RUBY=$(rbenv version-name)
LATEST_BUNDLER=$(curl -s https://rubygems.org/api/v1/versions/bundler/latest.json | cut -d'"' -f 4)
echo "Latest Bundler version: $LATEST_BUNDLER"

for RUBY_VERSION in $(rbenv versions --bare); do
  rbenv shell "$RUBY_VERSION"
  if $(gem list -i bundler); then
    CURRENT_BUNDLER=$(bundle --version | cut -d" " -f3)
    if [ "$CURRENT_BUNDLER" != "$LATEST_BUNDLER" ]; then
      echo "Updating $RUBY_VERSION Bundler from $CURRENT_BUNDLER"
      # Rubygems versions < 2.3.0 use a regex for `update` commands
      OLD_RUBYGEMS="abort if Gem::Version.new('$(gem --version)') >= Gem::Version.new('2.3.0')"
      if $(ruby -r rubygems -e "$OLD_RUBYGEMS"); then
        gem update ^bundler$
      else
        gem update bundler
      fi
    else
      echo "Ruby $RUBY_VERSION already has the latest Bundler"
    fi
  else
    echo "No Bundler version in $RUBY_VERSION"
    gem install bundler
  fi
done

echo "All done. Switching back to $CURRENT_RUBY"
rbenv shell "$CURRENT_RUBY"