Use filemod timestamp for version number for WordPress script registration, to bust cache.

wp-script-vers-filemod.php

<?php
/**
 * Use filemod timestamp for version number.
 *
 * For setting cache-busting version number in script registrations. Usage:
 * wp_register_script( 'handle', $file_url, array(), headecon_filemod_vers( $file_path ) );
 *
 * @param  string $path Path of script/style file
 * @return string $vers File-modification timestamp or WordPress version
 */
function headecon_filemod_vers( $path ) {
    $vers = '';

    if ( file_exists( $path ) ) {
        $vers = filemtime( $path );
    } else {
        $vers = get_bloginfo('version');
    }

    return $vers;
}