[gitg] Bump gtk+ dependency to 3.20

[Jesse van den Kieboom <jessevdk src gnome org>]

commit 1901580c31b0d80f87acfe7067df89f93fa938a2
Author: Jesse van den Kieboom <jessevdk gnome org>
Date:   Sun Jun 5 10:43:42 2016 +0200

    Bump gtk+ dependency to 3.20

 configure.ac |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 9b5082f..3ba56a9 100644
--- a/configure.ac
+++ b/configure.ac
@@ -129,7 +129,7 @@ AC_MSG_RESULT([$os_win32])
 AM_CONDITIONAL(OS_WIN32, test "$os_win32" = "yes")
 
 GLIB_REQUIRED_VERSION=2.38
-GTK_REQUIRED_VERSION=3.18.0
+GTK_REQUIRED_VERSION=3.20.0
 GTKSOURCEVIEW_REQUIRED_VERSION=3.10
 INTROSPECTION_REQUIRED=0.10.1
 LIBGIT2_GLIB_REQUIRED_VERSION=0.24.0