Emoji is small digital images or icons used to express ideas or emotions in electronic communication. When you update your website to WordPress to 4.2, a bunch of JS and CSS codes will be added to the site head. To see these js and CSS codes view page source of your site.

Look at the example image:

If you need, you can remove these codes from your WordPress website header. Copy below code and add it to functions.php file in your theme:

remove_action( 'wp_head', 'print_emoji_detection_script', 7 );
remove_action( 'wp_print_styles', 'print_emoji_styles' );

Save functions.php file and refresh your site.