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

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