Bundler.reset! nukes the global Definition.
CLI::auto_install performs the installation.

If requested by the environment variable and necessary, #auto_install will
automatically install any missing gems.
Autoinstall during the "show" command.
Autoinstall during the "binstubs" command.
Autoinstall during the "outdated" command.
Autoinstall for "bundle exec".
Autoinstall for "bundle open".
Autoinstall for "bundle console".
Autoinstall for "bundle license".
Autoinstall during "bundle clean".
Conditionally auto_install in initialize depending on command run
Merge pull request #2986 from gjaldon/autobundle

Option to auto-bundle before commands if `bundle check` is failing