fix: wallpaper not changing

Add new method to obtain GNOME version without breaking backwards compatibility.

closes #3

usr/local/bin/wallpaperctl

@@ -33,6 +33,8 @@ prepare() {
 get_gnome_version() {
     if [ -r "/usr/share/gnome/gnome-version.xml" ]; then
         gversion="$(grep -i "platform" /usr/share/gnome/gnome-version.xml | tr -d " <platform>" | tr -d "/")"
+    elif [ -e "/usr/bin/gnome-shell" ] && [ -x "/usr/bin/gnome-shell" ]; then
+       gversion="$(gnome-shell --version | cut -d' ' -f3 | cut -d'.' -f1)"
     fi
 }
 get_gnome_color_scheme() {