Bump copyright year.

src/js/pannellum.js

@@ -1,6 +1,6 @@
 /*
  * Pannellum - An HTML5 based Panorama Viewer
- * Copyright (c) 2011-2019 Matthew Petroff
+ * Copyright (c) 2011-2020 Matthew Petroff
  * 
  * Permission is hereby granted, free of charge, to any person obtaining a copy
  * of this software and associated documentation files (the "Software"), to deal