[valadoc] build: Bump valac requirement to 0.15.1 to match minimal driver support

[Rico Tzschichholz <ricotz src gnome org>]

commit d32e5a127eb2b5fa18da703c930018544be62161
Author: Rico Tzschichholz <ricotz ubuntu com>
Date:   Sat Jan 18 14:43:03 2014 +0100

    build: Bump valac requirement to 0.15.1 to match minimal driver support

 configure.ac |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index de36a90..0f30d25 100644
--- a/configure.ac
+++ b/configure.ac
@@ -22,7 +22,7 @@ PKG_PROG_PKG_CONFIG([0.21])
 ## Settings:
 ##
 
-VALA_REQUIRED=0.13.2
+VALA_REQUIRED=0.15.1
 LIBGEE_REQUIRED=0.8.0
 LIBGVC_REQUIRED=2.16
 GLIB_REQUIRED=2.24.0