Fix the copyright notice

Works made by civil servants come under Crown Copyright, according to
the Copyright, Designs and Patents Act 1988: see an explanation and link
to the legislation on The National Archives[1].

The copyright year is supposed to be the date the work was created,
which in this case dates back to the original commit in 2011:
8fa2772.

[1]: http://www.nationalarchives.gov.uk/information-management/re-using-public-sector-information/copyright/crown-copyright/

LICENCE

@@ -1,5 +1,5 @@
 The MIT License (MIT)
-Copyright (C) 2014 HM Government (Government Digital Service)
+Copyright (C) 2011 Crown copyright (Government Digital Service)
 
 Permission is hereby granted, free of charge, to any person obtaining a copy of
 this software and associated documentation files (the "Software"), to deal in