PHP updates to latest versions

In the last release of the Web Apps feature of Azure App Service, we have updated the PHP stacks to the latest available versions.

For information on the changes in the new versions, please see the change logs on the PHP website.

PHP version	Change log
5.6.31	http://www.php.net/ChangeLog-5.php#5.6.31
7.0.21	http://www.php.net/ChangeLog-7.php#7.0.21
7.1.7	http://www.php.net/ChangeLog-7.php#7.1.7