Update year in copyrright (celery#4788)

Author: mik-laj

docs/conf.py

@@ -13,7 +13,7 @@
     github_project='celery/celery',
     author='Ask Solem & contributors',
     author_name='Ask Solem',
-    copyright='2009-2017',
+    copyright='2009-2018',
     publisher='Celery Project',
     html_logo='images/celery_512.png',
     html_favicon='images/favicon.ico',