Add the Modernizr JS library for Internet Explorer 8 users and below.

functions.php

// remove the existing loading of html5shiv
remove_action( 'wp_head', 'genesis_html5_ie_fix' );

add_action( 'wp_enqueue_scripts', 'craigsimps_enqueue_modernizr' );
/**
 * Load the modernizr library, with built in html5shiv for users of IE8 or below.
 */
function craigsimps_enqueue_modernizr() {
	global $wp_scripts;
	wp_enqueue_script( 'craigsimps-modernizr', get_stylesheet_directory_uri() . '/assets/js/modernizr.min.js', array('jquery'), '2.8.3', true );
	$wp_scripts->add_data( 'craigsimps-modernizr', 'conditional', 'lt IE 8');
}