I understand that. And maybe there hasn't been much that needed updating since that. But the funny thing is, that DW keeps telling me that there is newer version available. The plugin manager tells me:
Status: installed, enabled, plugin
Download URL: github.com/haraldronge/groupmanager/archive/master.zip
Repository: github.com/haraldronge/groupmanager
Installed version: 2010-11-28
Your last update: Tue, 24 Mar 2015 12:42:54 +0100
Available version: 2013-05-27
Provides: Syntax
Compatible with: "Binky" (2013-12-08), "Weatherwax" (2013-05-10), "Adora Belle" (2012-10-13), "Angua" (2012-01-25), "Rincewind" (2011-05-25)
Similar to: groupmgr
So the installed version is indeed older than the last recognized and available update version from 2013. But when I click "update", nothing happens: afterwards the version is still the same from 2010.