Fix for oEmbed cache not clearing on update

Replaced do_shortcode() with $GLOBALS['wp_embed']->autoembed() in method delete_gist_transients in class GistPress. do_shortcode() does not does not process oEmbeds, which prevented the shortcode method from being executed and clearing the cache.

This is an improvement over my previous pull request, which used apply_filters( 'the_content' and therefore applied signficantly more filters than necessary.  Thank you to GaryJones for this improvement.