use modification time for version timestamp

It is more intuitive in my opinion if the version timestamp is the time when the file was edited and not the time when syncthing got a new version of the file.
You should also get better results when you want to sync your versions like described in https://github.com/syncthing/syncthing/issues/595 when versioning is enabled on multiple nodes
Please run this file through gofmt to clean up spacing etc.