fix HEROKU_PHP_DEFAULT_RUNTIME_VERSION default usage in platform.php
fix HEROKU_PHP_DEFAULT_RUNTIME_VERSION default usage in platform.php

only matters in local dev testing

emeth-/heroku-buildpack-php,emeth-/heroku-buildpack-php,heroku/heroku-buildpack-php,heroku/heroku-buildpack-php,heroku/heroku-buildpack-php,emeth-/heroku-buildpack-php,heroku/heroku-buildpack-php,emeth-/heroku-buildpack-php