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
This part would be simpler if you skipped the `now` and `versiondate` variables and then just `age := int64(time.Since(versionTime).Seconds())` (assuming age needs to be an int64, I didn't check that).