chore: Update copyright until date to 2021

INAL but this date range is purely decorational. If the date hasnt been
updated in a while, doesn't really matter AFAIK. MIT license and
copyright is still under effect if the license exists in the published source

LICENSE

@@ -1,6 +1,6 @@
 The MIT License
 
-Copyright (c) 2016-2018 Frappe Technologies Pvt. Ltd. <developers@frappe.io>
+Copyright (c) 2016-2021 Frappe Technologies Pvt. Ltd. <developers@frappe.io>
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal