Return unique filenames from Linux listener

so that we don't get duplicate entries for same file. This solves problem with
commonly used Linux editors (e.g. Vim, Emacs) that generate multiple inotify
events on file save.

http://schettino72.wordpress.com/2010/03/07/inotify-text-editors-emacs-vim/

lib/guard/listeners/linux.rb

@@ -60,6 +60,7 @@ module Guard
           update_last_event
 
           unless files.empty?
+            files.uniq!
             files.map! { |file| file.gsub("#{Dir.pwd}/", '') }
             callback.call(files)
             files.clear