manifest: Switch to cogl-1.18
manifest: Switch to cogl-1.18

Matching jhbuild for GNOME 3.12.

chergert/gnome-continuous,vrutkovs/vrutkovs.github.io,chergert/gnome-continuous,vrutkovs/vrutkovs.github.io,chergert/gnome-continuous,vrutkovs/vrutkovs.github.io