chore: fix LICENSE header so GH detects it

Author: chadlwilson

LICENSE.txt

@@ -1,10 +1,8 @@
 MIT License
 
-Warbler is provided under the terms of the MIT license.
-
-Warbler (c) 2013-2018 The JRuby Team
-Warbler (c) 2010-2013 Engine Yard, Inc.
-Warbler (c) 2007-2009 Sun Microsystems, Inc.
+Copyright (c) 2013-2018 The JRuby Team
+Copyright (c) 2010-2013 Engine Yard, Inc.
+Copyright (c) 2007-2009 Sun Microsystems, Inc.
 
 Permission is hereby granted, free of charge, to any person
 obtaining a copy of this software and associated documentation files