Check if delete_transient() exists to avoid error when upgrading from releases that don't define it.

git-svn-id: http://svn.automattic.com/wordpress/trunk@11356 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/update-core.php

@@ -250,7 +250,10 @@ function update_core($from, $to) {
 	$wp_filesystem->delete($from, true);
 
 	// Force refresh of update information
-	delete_transient('update_core');
+	if ( function_exists('delete_transient') )
+		delete_transient('update_core');
+	else
+		delete_option('update_core');
 
 	// Remove maintenance file, we're done.
 	$wp_filesystem->delete($maintenance_file);