Bump version to 1.18.20

debian/changelog

@@ -1,3 +1,9 @@
+dpkg (1.18.20) UNRELEASED; urgency=medium
+
+  *
+
+ -- Guillem Jover <guillem@debian.org>  Fri, 27 Jan 2017 06:51:35 +0100
+
 dpkg (1.18.19) unstable; urgency=medium
 
   [ Guillem Jover ]