Get HTTPS Everywhere using packages from experimental

This will bring the 3.x branch that contains many updated rules and also match
what TBB currently ships.

config/chroot_apt/preferences

@@ -299,7 +299,7 @@ Pin: release o=Debian,a=unstable
 Pin-Priority: 999
 
 Package: xul-ext-https-everywhere
-Pin: release o=Debian,a=unstable
+Pin: release o=Debian,a=experimental
 Pin-Priority: 999
 
 Package: xul-ext-monkeysphere