Move 'use strict' to file level

Change-Id: I1f0dae4aa168e77c112bcd3688c98b84e4ce5f36
diff --git a/dev/js/src/vc/docgroupref.js b/dev/js/src/vc/docgroupref.js
index 83ac244..9f1401a 100644
--- a/dev/js/src/vc/docgroupref.js
+++ b/dev/js/src/vc/docgroupref.js
@@ -2,6 +2,8 @@
  * A reference to another VC.
  * Inherits everything from jsonld
  */
+"use strict";
+
 define([
   'vc/jsonld',
   'vc/rewritelist',