configure.ac: bump version to 1.6.0
configure.ac: bump version to 1.6.0

Signed-off-by: Pekka Paalanen <0f6ae7001e34c37b02fb2c1c2573ddf8f442136a@collabora.co.uk>

Tarnyko/weston-xdg_surface_present,Fantu/compositor-spice,kwm81/weston,Gnurou/weston,xorgy/weston,eyolfson/weston,Gnurou/weston,Tarnyko/weston-xdg_surface_present,mchalupa/weston,mchalupa/weston,Fantu/compositor-spice,sir-murray/weston,udoprog/weston,giucam/weston,giucam/weston,jonnylamb/weston,krezovic/weston,jonnylamb/weston,xorgy/weston,udoprog/weston,sir-murray/weston,eyolfson/weston,Fantu/compositor-spice,kwm81/weston,krezovic/weston