Polling fallback message help

Motivated by: #284. [ci skip]
You're absolutely right! I was wrong, because I never saw that message (I use Linux) until I was testing older versions. (Sass comes with Listen 1.3.1, and that "bracket" message has been since 1.0.0.).

I think you can drop the reference to Bundler, because if people successfully install a newer version, it means they have Ruby 1.9.3 at least. If the installation fails and they don't know how to upgrade Ruby, Sass will use it's builtin version. So just mentioning installing a never version is more than sufficient (it will get people to the next step or next problem).

The Listen README is so complex already (my fault), the less text, the better.

Personally, I'd just include the `wdm` gem in Listen directly, anyway - just to reduce the documentation. The reason not to is that Sass forces polling anyway (supposedly because of better performance on Windows) - and people on other systems probably wouldn't want the extra gem dependency. There a