PHP 5.x reaches the end of life by the end of 2018

Description

PHP 5.x reaches the end of life by the end of 2018 so we need to upgrade our PHP clients, servers to use PHP 7.x as the default.

Ref: http://php.net/supported-versions.php

openapi-generator version

I suggest we do it as part of the 4.0.0 release (next major release)

Suggest a fix/enhancement

If anyone wants to help on this, please reply to let us know.