Update for sphinx internal structure change layout

As reported upstream in sphinx:
https://github.com/sphinx-doc/sphinx/pull/2396

It was a structural change in sphinx-1.4. I'm not sure if this is the
proper fix, but it does let the doc build pass, in my testing.

Originally reported at
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=830178.

Signed-off-by: Nishanth Aravamudan <nish.aravamudan@canonical.com>
diff --git a/doc/build/templates/genindex.mako b/doc/build/templates/genindex.mako
index 0a25b67..2f45f79 100644
--- a/doc/build/templates/genindex.mako
+++ b/doc/build/templates/genindex.mako
@@ -21,7 +21,7 @@
         numcols = 1
         numitems = 0
     %>
-% for entryname, (links, subitems) in entries:
+% for entryname, (links, subitems, dummy) in entries:
 
 <dt>
     % if links: