<?php
add_action( 'wp_enqueue_scripts', 'my_theme_enqueue_styles' );
function my_theme_enqueue_styles() {
  $parenthandle = 'twentyfifteen-style';
  $theme        = wp_get_theme();
  wp_enqueue_style( $parenthandle,
    get_template_directory_uri() . '/style.css',
    array(),  // If the parent theme code has a dependency, copy it to here.
    $theme->parent()->get( 'Version' )
  );
  wp_enqueue_style( 'child-style',
    get_stylesheet_uri(),
    array( $parenthandle ),
    $theme->get( 'Version' ) // This only works if you have Version defined in the style header.
  );
}