Update plugin license to GPLv3

Author: BenjaminLawson

pdfjs-viewer.php

@@ -6,7 +6,7 @@
 Version: 1.3
 Author: Ben Lawson
 Author URI: http://byterevel.com/
-License: GPLv2
+License: GPLv3
 */
 
 
@@ -81,4 +81,4 @@ function include_pdfjs_media_button_js_file() {
     wp_enqueue_script('media_button', plugins_url().'/pdfjs-viewer-shortcode/pdfjs-media-button.js', array('jquery'), '1.0', true);
 }
 
-?>
+?>