configure.ac: Bump master version to 1.5.90

Post release bump.  The master branch version is always x.y.90,
where x.y is the most recent stable branch.  This lets other packages
rely on git master as opposed to 1.5.0.

configure.ac

@@ -1,6 +1,6 @@
 m4_define([weston_major_version], [1])
 m4_define([weston_minor_version], [5])
-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])