Log Message:
www/ruby-loofah: update to 2.4.0

Update ruby-loofah to 2.4.0.

## 2.4.0 / 2019-11-25

### Features

* Allow CSS property `max-width` [#175] (Thanks, @bchaney!)
* Allow CSS sizes expressed in `rem` [#176, #177]
* Add `frozen_string_literal: true` magic comment to all `lib` files. [#118]