Revert "debian/gbp.conf: Set debian-branch to 1.8.y"

This reverts commit 51ac850da8.

debian/gbp.conf

@@ -3,7 +3,7 @@
 ;preexport = ./prepare-release pre-export
 prebuild = ./prepare-release pre-build
 postbuild = ./prepare-release post-build
-debian-branch = 1.8.y
+debian-branch = master
 debian-tag = %(version)s
 export-dir = ../build-area
 sign-tags = True