WooCommerce - Restore "Free!" for free simple products on WooCommerce 3.0.

## functions.php
<?php
/**
 * Restore single product "Free!" on WooCommerce 3.0.
 *
 * @param  string     $price   Price HTML.
 * @param  WC_Product $product Product instance.
 * @return string.
 */
function my_wc_custom_get_price_html( $price, $product ) {
	if ( $product->get_price() == 0 ) {
		if ( $product->is_on_sale() && $product->get_regular_price() ) {
			$regular_price = wc_get_price_to_display( $product, array( 'qty' => 1, 'price' => $product->get_regular_price() ) );

			$price = wc_format_price_range( $regular_price, __( 'Free!', 'woocommerce' ) );
		} else {
			$price = '<span class="amount">' . __( 'Free!', 'woocommerce' ) . '</span>';
		}
	}

	return $price;
}

add_filter( 'woocommerce_get_price_html', 'my_wc_custom_get_price_html', 10, 2 );