configure.ac: bump version to 1.6.90
configure.ac: bump version to 1.6.90

Master is open for new features again.

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

eyolfson/wayland,mchalupa/wayland,sir-murray/wayland,tulcod/wayland,tulcod/wayland,bitfactor/wayland,mchalupa/wayland,tulcod/wayland,eyolfson/wayland,abooij/wayland,sir-murray/wayland,abooij/wayland,sir-murray/wayland,bitfactor/wayland,bitfactor/wayland,mchalupa/wayland,abooij/wayland,eyolfson/wayland