add stretch security repos (now that they are available)

templates/os/sources/sources.list.erb

@@ -20,13 +20,10 @@ deb-src  http://debian.stanford.edu/ubuntu <%= @lsbdistcodename %>-updates main
 deb     http://debian.stanford.edu/debian <%= @lsbdistcodename %> main non-free contrib
 deb-src http://debian.stanford.edu/debian <%= @lsbdistcodename %> main non-free contrib
 <% end -%>
-<% if @lsbdistcodename == 'squeeze' -%>
-
-# The long-term support archive, which provides extended security support.
-deb     http://debian.stanford.edu/debian <%= @lsbdistcodename %>-lts main non-free contrib
-deb-src http://debian.stanford.edu/debian <%= @lsbdistcodename %>-lts main non-free contrib
-<% end -%>
-<% if (@lsbdistcodename == 'squeeze') or (@lsbdistcodename == 'wheezy') or (@lsbdistcodename == 'jessie') -%>
+   if (@lsbdistcodename == 'wheezy')
+   or (@lsbdistcodename == 'jessie')
+   or (@lsbdistcodename == 'stretch')
+-%>
 
 # The archive that publishes updates for security vulnerabilities.
 deb     http://debian.stanford.edu/debian-security <%= @lsbdistcodename %>/updates main non-free contrib