release: prepare for 3.5.91

NEWS

+3.5.91
+======
+
+Add support for vertically oriented spinbuttons
+Add support for a selection-menu button in selection mode
+Set scrolled windows background for normal and backdrop states
+Use new tooltip radius and alpha style properties for GTK2
+Use new range warp slider style property for GTK2
+Fix Chrome/Chromium custom menu items for GTK2
+Fix wrong text color in app notification widgets
+Fix tiny padding in Nautilus pathbars
+Fix stray background color below checkboxes in content views
+Synchronize HighContrast icons from the symbolic theme
+
 3.5.90.1
 ========
 

configure.ac

 dnl Process this file with autoconf to produce a configure script.
 AC_PREREQ(2.53)
 
-AC_INIT([gnome-themes-standard], [3.5.90.1],
+AC_INIT([gnome-themes-standard], [3.5.91],
         [http://bugzilla.gnome.org/enter_bug.cgi?product=gnome-themes-standard])
 
 AM_INIT_AUTOMAKE([1.9 tar-ustar foreign dist-xz no-dist-gzip])