Merge bitcoin#11318: Put back inadvertently removed copyright notices

d552ed6 Put back inadvertently removed copyright notices (Paul Berg)

Pull request description:

  In an abundance of caution this restores "Bitcoin Developers" to the COPYING file in
  case there were contributors before that point in time that would object to the
  current label.  It's harmless and more pedantically correct.

  (Change extracted from the Bitcoin-abc github)

Tree-SHA512: ea71645fbca5548bcae750b9bb7c7cc56791d8c8d2574b778fa1666e09abad23b30fcb434f0d92c52a2eb7d7e89caa32695dff3a5a99a2ab43ed0f3636436677

COPYING

@@ -1,6 +1,7 @@
 The MIT License (MIT)
 
 Copyright (c) 2009-2017 The Bitcoin Core developers
+Copyright (c) 2009-2017 Bitcoin Developers
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal