gdk-pixbuf: update to version 2.31.1
pygobject: update to version 3.13.91
vala: Cleanup.
Merge pull request #173 from nacho/wip/gdk-pixbuf2.31.1

gdk-pixbuf: update to version 2.31.1
Merge pull request #174 from nacho/wip/pygobject3.13.91

pygobject: update to version 3.13.91