configure.ac: bump version to 1.6.91 for the alpha release
configure.ac: bump version to 1.6.91 for the alpha release

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