Enqueue scripts in the footer

From http://codex.wordpress.org/Function_Reference/wp_register_script:

<?php wp_register_script( $handle, $src, $deps, $ver, $in_footer ); ?>

This means you need to add the $deps and $ver values to wp_register_script() in order to set $in_footer. Like this:

wp_register_script(
    'justifiedGallery', 
    plugins_url('js/jquery.justifiedGallery.min.js', __FILE__),
    false, // or array(), or array('jquery') if this depends on jQuery
    '1.0', // or your plugin version, or the version of the js file
    true   // $in_footer
);