configure.ac: Bump version to 1.3.93
configure.ac: Bump version to 1.3.93

This is the release candidate.

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