build: Bump required GTK+ version

For the new API for handling RTL icons.

https://bugzilla.gnome.org/show_bug.cgi?id=737211
ev-history-action: Use the new support for RTL icons in GtkIconTheme

https://bugzilla.gnome.org/show_bug.cgi?id=737211
ev-toolbar: Use margin-end instead of margin-right

So we not needs to handle RTL margin ourselves.

https://bugzilla.gnome.org/show_bug.cgi?id=737211