debian/gbp.conf: extend default configuration

Signed-off-by: Héctor Orón Martínez <hector.oron@collabora.co.uk>

debian/gbp.conf

 [DEFAULT]
-debian-tag = %(version)s
+debian-tag = apertis/%(version)s
+debian-branch = master
+pristine-tar = True
 
 [dch]
 ignore-branch = True