open-vcdiff: update to 0.8.4

wget is ignoring "Content-Disposition" http header, that's why I've added 'downloadfilename' in SRC_URI.

Note: Googlecode discarded their download section and moved to Googledrive.
http://google-opensource.blogspot.fr/2013/05/a-change-to-google-code-download-service.html



Signed-off-by: Matthieu Crapet <Matthieu.Crapet@ingenico.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>