Update standard copyright header for new year (go-gitea#3289)

Author: techknowlogick

CONTRIBUTING.md

@@ -252,7 +252,7 @@ be reviewed by two maintainers and must pass the automatic tests.
 Code that you contribute should use the standard copyright header:
 
 ```
-// Copyright 2017 The Gitea Authors. All rights reserved.
+// Copyright 2018 The Gitea Authors. All rights reserved.
 // Use of this source code is governed by a MIT-style
 // license that can be found in the LICENSE file.
 ```