Just update copyright

- Year updated.
- Mention of "project contributors" in the copyright.

Thanks to all who contributed the code to the project!

go-playground.el

@@ -1,11 +1,12 @@
 ;;; go-playground.el --- Local Golang playground for short snippets.
 
-;; Copyright (C) 2015-2019  Alexander I.Grafov
+;; Copyright (C) 2015-2021 Alexander I.Grafov and the project
+;; contibutors.
 
 ;; Author: Alexander I.Grafov <[email protected]>
 ;; URL: https://github.com/grafov/go-playground
 ;; Keywords: tools, golang
-;; Version: 1.7
+;; Version: 1.7.1
 ;; Package-Requires: ((emacs "24") (go-mode "1.4.0") (gotest "0.13.0"))
 
 ;; This program is free software; you can redistribute it and/or modify