Drop support for Gtk-3.16

Styles for 3.18 also works for 3.16. From now on we don't expilicitly support Gtk-3.16

Author: khurshid-alam

README.md

@@ -52,7 +52,7 @@ If you change any assets, you'll need to regenerate the `gtk.gresource.xml` and
 
 ### Requirements
 
-GTK+ 3.16 or above
+GTK+ 3.18 or above
 
 Murrine theme engine