configure.ac: Bump version to 1.5.91 for the alpha release
configure.ac: Bump version to 1.5.91 for the alpha release

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

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