Update copyright notice

Ports https://github.com/twbs/bootstrap/pull/19936 to bootstrap-sass

LICENSE

@@ -1,6 +1,7 @@
 The MIT License (MIT)
 
-Copyright (c) 2013-2016 Twitter, Inc
+Copyright (c) 2011-2016 Twitter, Inc
+Copyright (c) 2011-2016 The Bootstrap Authors
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal