add_filter( 'style_loader_src', 'remove_src_version' );
function remove_src_version ( $src ) {
  global $wp_version;
  $version_str = '?ver='.$wp_version;
  $version_str_offset = strlen( $src ) - strlen( $version_str );

  if( substr( $src, $version_str_offset ) == $version_str )
    $without_ver = substr( $src, 0, $version_str_offset );

  $file_path = str_replace(get_stylesheet_directory_uri(), get_stylesheet_directory(), $without_ver);

  $file_path_alt = str_replace(get_template_directory_uri(), get_template_directory(), $without_ver);

  if (file_exists($file_path)) {
    return $without_ver.'?ver='.filemtime($file_path);
  } elseif (file_exists($file_path_alt)) {
    return $without_ver.'?ver='.filemtime($file_path_alt);
  }
  else return $without_ver;

}