[caribou] Fix PyGIWarning warnings in preferences_window.py

[Daiki Ueno <dueno src gnome org>]

commit d5d0dc7be9663643f5e1ae7fec7241325cf7cb43
Author: Parag Nemade <pnemade redhat com>
Date:   Fri Apr 8 11:14:33 2016 +0530

    Fix PyGIWarning warnings in preferences_window.py
    
    https://bugzilla.gnome.org/show_bug.cgi?id=764764

 caribou/settings/preferences_window.py |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/caribou/settings/preferences_window.py b/caribou/settings/preferences_window.py
index 541ecb5..350ab59 100644
--- a/caribou/settings/preferences_window.py
+++ b/caribou/settings/preferences_window.py
@@ -21,6 +21,8 @@
 from caribou.settings.setting_types import *
 
 from gi.repository import GObject
+gi.require_version('Gtk', '3.0')
+gi.require_version('Gdk', '3.0')
 from gi.repository import Gdk
 from gi.repository import Gtk