build: prepare for 2.91.91

NEWS

+2.91.91
+=======
+
+Big revamp of the Adwaita engine to match 3.0 designs.
+
+Update include:
+- checkboxes/radiobuttons
+- notebook tabs shape
+- expanders
+- arrows
+- range sliders
+- progressbar
+- scrollbars
+- insensitive icons
+- new color palette
+
+and probably even more.
+
 2.91.90.1
 =========
 

configure.ac

 dnl Process this file with autoconf to produce a configure script.
 AC_PREREQ(2.53)
 
-AC_INIT([gnome-themes-standard], [2.91.90.1],
+AC_INIT([gnome-themes-standard], [2.91.91],
         [http://bugzilla.gnome.org/enter_bug.cgi?product=gnome-themes-standard])
 
 AM_INIT_AUTOMAKE([1.9 tar-ustar foreign dist-bzip2 no-dist-gzip])