Install obfs4proxy from deb.torproject.org on Jessie as well.

The package in Jessie is older than the one from deb.tpo, that we currently ship
in Tails 1.x, so let's fetch it from the same place.

Will-Fix: #10605

config/chroot_apt/preferences

@@ -126,6 +126,10 @@ Package: linux-kbuild-3.16
 Pin: release o=Debian,n=jessie
 Pin-Priority: 999
 
+Package: obfs4proxy
+Pin: release o=TorProject,n=obfs4proxy
+Pin-Priority: 990
+
 Package: ttdnsd
 Pin: release o=TorProject,a=unstable
 Pin-Priority: 999