Remove old announcement before setting new one

This prevents the case where the old announcement had a url set and the
new one does not (Redis would keep the existing url).

mutalyzer/announce.py

@@ -18,6 +18,7 @@ def set_announcement(body, url=None):
     if url:
         announcement['url'] = url
 
+    unset_announcement()
     client.hmset('announcement', announcement)