Defer parsing javascript

Hi everybody,
I am trying to speed up my sites and came across “defer parsing javascript”.
I read a lot about it and understand you can put a code in your funtions.php.
I did that, in the child theme of my theme, but that didn’t work.
This is the code I used:

// Defer jQuery Parsing using HTML5’s defer property
if (!(is_admin() )) {
function defer_parsing_of_js ( $url ) {
if ( FALSE === strpos( $url, ‘.js’ ) ) return $url;
if ( strpos( $url, ‘jquery.js’ ) ) return $url;
return "$url’ defer ";
}
add_filter( ‘clean_url’, ‘defer_parsing_of_js’, 11, 1 );
}

The admin part is to let texteditors in the admin work alright.
But also without that part I see no difference in the gtmetrix score.
I also tried the WP deferred javascripts plugin, but that didn’t solve it either.
The defer parsing in Gtmetrix is still at 0.
I also changed themes to the Twenty Eleven theme to see if it is theme related, but that didn’t solve it either.
What am I missing here? Anybody has an idea?

Johanna