Run on modifications instead of changes.

This is basically mimicking changes that have been made to several guard gems already, triggering on modifications instead of changes which leads to faulty behaviour when using vim. (guard/guard#495)

I am not sure if we need to change this in the backward compatibilty branch in rake.rb#L44 as well, I guess probably not.

PS: Oh the cleanup of trailing whitespace was done automatically by vim ;)