Revert "Upgrade to Debian 9 (a.k.a. stretch)"

Oops. I accidentally pushed to master...

This reverts commit 29443be0516152967061adb3eda60774daf4d380.

Change-Id: I853bf174dfff592ff6de0928adc88037f7e4a269
diff --git a/vm/setup_apt.sh b/vm/setup_apt.sh
index a2c2d28..d31a372 100755
--- a/vm/setup_apt.sh
+++ b/vm/setup_apt.sh
@@ -7,8 +7,8 @@
 
 # Set up contrib and non-free so we can install some more interesting programs.
 cat > /etc/apt/sources.list.d/contrib.list <<EOT
-deb  http://ftp.us.debian.org/debian/ stretch contrib non-free
-deb-src  http://ftp.us.debian.org/debian/ stretch contrib non-free
+deb  http://ftp.us.debian.org/debian/ jessie contrib non-free
+deb-src  http://ftp.us.debian.org/debian/ jessie contrib non-free
 EOT
 
 # Get a list of the latest packages.