Drop use of create_function

In https://plugins.trac.ww.wp.xz.cn/browser/wd-instagram-feed/trunk/wd-instagram-feed.php#L421 , the plugin is using create_function which is deprecated in upcoming PHP 7.2.0 ( see http://php.net/manual/en/function.create-function.php ).

Happy to do the PR for this, but didn’t immediately see a GH link.