[gnome-builder] html-preview: silence a warning

[Christian Hergert <chergert src gnome org>]

commit b4876adb9e494083dfb3a88e0775868fecc24a4f
Author: Christian Hergert <christian hergert me>
Date:   Thu Dec 24 14:14:28 2015 -0800

    html-preview: silence a warning

 .../html-preview/html_preview_plugin/__init__.py   |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/plugins/html-preview/html_preview_plugin/__init__.py 
b/plugins/html-preview/html_preview_plugin/__init__.py
index e0ffa48..4454ccc 100644
--- a/plugins/html-preview/html_preview_plugin/__init__.py
+++ b/plugins/html-preview/html_preview_plugin/__init__.py
@@ -24,6 +24,7 @@ from gettext import gettext as _
 import gi
 import os
 
+gi.require_version('Gtk', '3.0')
 gi.require_version('Ide', '1.0')
 gi.require_version('WebKit2', '4.0')