configure.ac: Bump version to 1.5.91 for the alpha release

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
diff --git a/configure.ac b/configure.ac
index bc5c88a..d282c76 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1,6 +1,6 @@
 m4_define([weston_major_version], [1])
 m4_define([weston_minor_version], [5])
-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])