Fix for oEmbed cache not clearing on update.

Patch for issue #34

Replaced do_shortcode() with apply_filters( 'the_content' 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.

Improved as per @GaryJones by using
```
do_shortcode( $GLOBALS['wp_embed']->autoembed( $post_before->post_content ) );
do_shortcode( $GLOBALS['wp_embed']->autoembed( $post_after->post_content ) );
```

Corrected `$attrs` instead of `$rawattr` as pointed out by @bradyvercher in comments on #34