Upgrade to tor 0.3.4.7-rc (refs: #15772).

There's now a RC of 0.3.4 packaged for Stretch, let's use it instead of
an older Git snapshot.

config/APT_snapshots.d/torproject/serial

-2018081602
+2018090101

config/chroot_apt/preferences

@@ -97,7 +97,7 @@ Pin: origin deb.tails.boum.org
 Pin-Priority: 999
 
 Package: tor tor-geoipdb
-Pin: release o=TorProject,n=tor-nightly-0.3.4.x-stretch
+Pin: release o=TorProject,n=tor-experimental-0.3.4.x-stretch
 Pin-Priority: 999
 
 Package: virtualbox*

config/chroot_sources/torproject.chroot

 deb http://deb.torproject.org/torproject.org stretch main
-deb http://deb.torproject.org/torproject.org tor-nightly-0.3.4.x-stretch main
+deb http://deb.torproject.org/torproject.org tor-experimental-0.3.4.x-stretch main