Rework the explanation of watching in the readme
Rename watch_via option to watch_method

It reads a bit better to me.
Don't fallback to polling

If the user has specified `Spring.watch_method = :listen`, but listen is
not available, just let it throw a `LoadError`.
Listen version must be ~> 1.0

Pre-1.0 support was broken anyway, and I don't want to maintain it.
Add changelog item for the Spring.watch_method= config option