COPYING: Update year range to 2016 (close #12621)

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COPYING
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Copyright (c) 2003-2015 Eelco Dolstra and the Nixpkgs/NixOS contributors
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Copyright (c) 2003-2016 Eelco Dolstra and the Nixpkgs/NixOS contributors
Permission is hereby granted, free of charge, to any person obtaining
a copy of this software and associated documentation files (the