Bump version to 1.18.4

debian/changelog

@@ -1,3 +1,9 @@
+dpkg (1.18.4) UNRELEASED; urgency=low
+
+  *
+
+ -- Guillem Jover <guillem@debian.org>  Mon, 21 Sep 2015 07:48:56 +0200
+
 dpkg (1.18.3) unstable; urgency=medium
 
   [ Guillem Jover ]