colornames: fix copyright year.

These files were added in 2015:
https://go-review.googlesource.com/#/c/13181/

Presumably they were copy/pasted from something written in 2011.

Change-Id: I3defb4e02d03beb835e8a16c89bcae44c292e914
Reviewed-on: https://go-review.googlesource.com/14487
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

colornames/colornames.go

@@ -1,4 +1,4 @@
-// Copyright 2011 The Go Authors. All rights reserved.
+// Copyright 2015 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 

colornames/colornames_test.go

@@ -1,4 +1,4 @@
-// Copyright 2011 The Go Authors. All rights reserved.
+// Copyright 2015 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 

colornames/gen.go

@@ -1,4 +1,4 @@
-// Copyright 2011 The Go Authors. All rights reserved.
+// Copyright 2015 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.