build: Post release bump to 1.5.7
build: Post release bump to 1.5.7

GNOME/clutter-gst,ystreet/clutter-gst,lubosz/clutter-gst,lubosz/clutter-gst,ystreet/clutter-gst,lubosz/clutter-gst,ystreet/clutter-gst,GNOME/clutter-gst,GNOME/clutter-gst,GNOME/clutter-gst,ystreet/clutter-gst