bump add_filter "do_shortcode" to priority 11, follow core

See https://github.com/WordPress/WordPress/blob/3.8.1/wp-includes/shortcodes.php#L388

Because shortcode_unautop() is seriously lacking [1], sufficient cause
exists to replace this function with a custom plugin until core gets
better. Putting everything on priority 10 in GC creates unnecessary
filter ordering problems when replacing built-in shortcode_unautop(),
meaning one would also have to remove and re-add GC's "do_shortcode"
filter, which must always run after "shortcode_unautop". Might as well
do the right thing upstream.

[1]: http://core.trac.wordpress.org/ticket/14050