bootstrap.conf: require bison 3.6 or later

Signed-off-by: Daiki Ueno <ueno@gnu.org>
diff --git a/bootstrap.conf b/bootstrap.conf
index 45e9689..c8a85b8 100644
--- a/bootstrap.conf
+++ b/bootstrap.conf
@@ -42,7 +42,7 @@
 autoconf   2.62
 automake   1.11.1
 git        1.4.4
-bison      -
+bison      3.6
 "
 
 GTKDOCIZE=$(which gtkdocize 2>/dev/null)