Polling fallback message help

Motivated by: #284. [ci skip]
Note: I did see `[Listen warning]` on this line of master branch:

https://github.com/guard/listen/blob/2dd0b7e1f3d4de4cf720a5b5c5c1010c4d01652a/lib/listen/adapter.rb#L38

Looks like that text may show in the latest version of Listen depending on context/message? If that's true, then here's how I might suggest we word things:

> …
> * Using [Sass](http://sass-lang.com/), which includes an _**ancient**_ (the “dinosaur” type of ancient) version of the Listen gem.

> Possible solutions:

> * …
> * Sass users: Install the latest version of Listen and try again (Bundler, as mentioned above, is recommended in this case).

If I'm wrong about `[Listen warning]` then maybe we could word the above like:

> * Sass users: You’ll know you’re running an **ancient** version of Listen if you see `[listen warning]` text (in square brackets); if you see this text, first install the latest version of Listen and try again (Bundler, as mentioned above, is recommended in this case).

Should I drop the note about Bundler? On a Mac, that's the direction I would head, but I also don't want to complicate the verbiage here.