Update memberships.wml to remove Desktop Linux Consortium and add Open Source Initiative.
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Removed Desktop Linux Consotrium, as organization no longer exists (https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=861781#5).
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Added Open Source Initiative, which is included on the list of affiliates on the OSI web page (https://opensource.org/affiliates/list and https://www.debian.org/News/2012/20120330).