[rygel] build: Bump vala version

[Jens Georg <jensgeorg src gnome org>]

commit bd4d3800297e8d1d0442477de84ccee3f3ae3a18
Author: Jens Georg <jensg openismus com>
Date:   Fri Jul 12 10:19:10 2013 +0200

    build: Bump vala version
    
    Necessary for search_ns () in libxml2 vapi from commit 6a481ee
    
    https://bugzilla.gnome.org/show_bug.cgi?id=704066

 configure.ac |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 001904c..3e67688 100644
--- a/configure.ac
+++ b/configure.ac
@@ -31,7 +31,7 @@ dnl Required versions of library packages
 dnl Not all of these are actually used, depending on the configure options.
 GLIB_REQUIRED=2.31.13
 LIBXML_REQUIRED=2.7
-VALA_REQUIRED=0.18.0
+VALA_REQUIRED=0.20.0
 VALADOC_REQUIRED=0.2
 GSSDP_REQUIRED=0.13.0
 GUPNP_REQUIRED=0.19.0