Add blank line to output for FileDownloader.

The VcsDownloader outputs a blank line between each dependency:

https://github.com/composer/composer/blob/745dcbce3317f7119575c39cef2cb601f9c5ffcf/src/Composer/Downloader/VcsDownloader.php#L80

This write makes output consistent.
Merge pull request #3122 from garthkerr/patch-1

Add blank line to output for FileDownloader.