Update copyright notices to 2020

debian/copyright

@@ -13,7 +13,7 @@ Files:
  debian/*
 Copyright:
  2005-2008 Mario Iseli <admin@marioiseli.com>,
- 2010-2017 Christoph Biedl <debian.axhn@manchmal.in-ulm.de>
+ 2010-2020 Christoph Biedl <debian.axhn@manchmal.in-ulm.de>
 License: GPL-2+
 
 License: GPL-2+