configure.ac: bump version to 1.6.90

Open master for new features again.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

configure.ac

 m4_define([weston_major_version], [1])
 m4_define([weston_minor_version], [6])
-m4_define([weston_micro_version], [0])
+m4_define([weston_micro_version], [90])
 m4_define([weston_version],
           [weston_major_version.weston_minor_version.weston_micro_version])