Add back wheezy-updates now that it's mirrored properly

This was only missing because our mirror script didn't know about
it.  That's now been fixed, so add wheezy-updates back into the
default sources.list for wheezy.

Remove some comments that don't make sense in the generated file.

templates/os/sources/sources.list.erb

@@ -22,18 +22,14 @@ deb-src http://debian.stanford.edu/debian <%= lsbdistcodename %> main non-free c
 <% end -%>
 <% if (lsbdistcodename == 'squeeze') or (lsbdistcodename == 'wheezy') -%>
 
-# The additional archives below only exist for released versions of Debian
-# that have not been retired.
-
 # The archive that publishes updates for security vulnerabilities.
 deb     http://debian.stanford.edu/debian-security <%= lsbdistcodename %>/updates main non-free contrib
 deb-src http://debian.stanford.edu/debian-security <%= lsbdistcodename %>/updates main non-free contrib
-<% if lsbdistcodename == 'squeeze' -%>
 
-# The updates Debian archive and source tree.  This is a replacement for
-# the volatile archive and contains packages that change more frequently
-# than stable point releases but are still carefully vetted.
+# The updates Debian archive and source tree.  This contains packages that
+# change more frequently than stable point releases but are still
+# carefully vetted.  (It's the staging area for the next stable point
+# release.)
 deb     http://debian.stanford.edu/debian <%= lsbdistcodename %>-updates main non-free contrib
 deb-src http://debian.stanford.edu/debian <%= lsbdistcodename %>-updates main non-free contrib
 <% end -%>
-<% end -%>