Use a Listen stable API

Hi, this PR simplifies 749f07c a bit!

The `Listen.to` method was actually there before v1.0 and was already allowing you to listen to multiple directories! Cheers!

I've also updated the README since Listen now depends on the OS-specific adapters for Mac OS X 10.6+, Linux, *BSD and Windows [directly in its gemspec](https://github.com/guard/listen/blob/master/listen.gemspec#L18-L20).