Polling fallback message help

Motivated by: #284. [ci skip]
2 things about Sass:
- it's not only old, it's ancient (the "dinosaur" type of ancient)
- people will know if they have the ancient version by the `[listen warning]` text (in square brackets) - if they do, they should first install the latest version of Listen and try again

I'm just mentioning this in case you feel it's important - personally, I'm fine with the way it is right now.