Update the debian changelog for 0.18

Also include the changelog entry for 0.17.1, which hadn't been added at
the time of release.

Author: gammafunk

crawl-ref/source/debian/changelog

@@ -1,3 +1,15 @@
+crawl (2:0.18.0-1) UNRELEASED; urgency=low
+
+  * New upstream release.
+
+-- gammafunk <gammafunk@gmail.com>  Wed, 04 May 2016 13:27:07 -0500
+
+crawl (2:0.17.1-1) UNRELEASED; urgency=low
+
+  * New upstream release.
+
+-- gammafunk <gammafunk@gmail.com>  Thu, 10 Dec 2015 05:09:43 -0600
+
 crawl (2:0.17.0-1) UNRELEASED; urgency=low
 
   * New upstream release.