def remove_vendor_bundle
    vendor_bundle = self.app_path.join("vendor").join("bundle")
    if vendor_bundle.exist?
      warn(<<~WARNING)
        Removing `vendor/bundle`.
        Checking in `vendor/bundle` is not supported. Please remove this directory
        and add it to your .gitignore. To vendor your gems with Bundler, use
        `bundle pack` instead.
      WARNING
      vendor_bundle.rmtree
    end
  end
remove `vendor/bundle` that comes from the git repo
in case there are native ext.
users should be using `bundle pack` instead.
https://github.com/heroku/heroku-buildpack-ruby/issues/21