[gnome-continuous] manifest: Bump up the tag for libpng

[Emmanuele Bassi <ebassi src gnome org>]

commit 20d6c31bb00bfc9847b433a8f588f399d25edd93
Author: Emmanuele Bassi <ebassi gnome org>
Date:   Thu Jan 7 16:39:36 2016 +0000

    manifest: Bump up the tag for libpng
    
    GNOME Photos depends on libpng 1.6.x, and there have been various
    CVEs raised for older versions of the library.

 manifest.json |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/manifest.json b/manifest.json
index ffab581..acbb71a 100644
--- a/manifest.json
+++ b/manifest.json
@@ -321,7 +321,7 @@
                 "setuid": ["/runtime/usr/libexec/dbus-daemon-launch-helper"]},
 
                {"src": "git:git://libpng.git.sourceforge.net/gitroot/libpng/libpng",
-                "tag": "v1.5.18"},
+                "tag": "v1.6.20"},
 
                {"src": "fd:cairo",
                 "config-opts": ["--disable-interpreter"]},