build: bump version to 1.4.18

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
diff --git a/configure.ac b/configure.ac
index 27e0b10..2ba5cec 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1,5 +1,5 @@
 
-AC_INIT([iptables], [1.4.17])
+AC_INIT([iptables], [1.4.18])
 
 # See libtool.info "Libtool's versioning system"
 libxtables_vcurrent=10