def require_gems
      Jekyll::External.require_with_graceful_fail(
        site.gems.select { |plugin| plugin_allowed?(plugin) }
      )
    end
def require_gems
      Jekyll::External.require_with_graceful_fail(
        site.gems.select { |plugin| plugin_allowed?(plugin) }
      )
    end
Require each of the gem plugins specified.

 Returns nothing.