Remove empty line and PHP closing tag that couse PHP warning. fixed #4

> Warning: Cannot modify header information - headers already sent by (output started at /path/to/WordPress/wp-content/plugins/wp-highlightjs/wp_highlight.js.php:248) in [snip]

This PR will stop the warning avobe.
<del>The warning comes up on `WP_DEBUG = true`.</del>
The warning comes up on both `WP_DEBUG = true / false` except `WP_DEBUG = false` && `display_errors` is `Off`.