How to change src link in YouTube?

function add_embed_filter($html) {
    return '<div class="js-video widescreen">'.str_replace("?feature=oembed", "?html5=1", $html).'</div>';
}
add_filter( 'embed_oembed_html', 'add_embed_filter', 50, 1);

Of course if WordPress or Youtube change how this url is constructed you will have to adapt your code accordingly, but I tested it, and this will get you there with WordPress 3.8.1. I also did not test how this works for a user who does not support HTML5 and depending on your use case, you may wish to test to ensure that this code will support them.