configure.ac: bump to version 3.0.91 for the alpha release

configure.ac

 m4_define([weston_major_version], [3])
 m4_define([weston_minor_version], [0])
-m4_define([weston_micro_version], [90])
+m4_define([weston_micro_version], [91])
 m4_define([weston_version],
           [weston_major_version.weston_minor_version.weston_micro_version])
 m4_define([libweston_major_version], [4])