Use exact version for default_version
Use exact version for default_version

- The buildpack can't handle floating default_versions

Co-authored-by: David Freilich <0106e5ee349bb58d7c51800b6acf5b57cf80aef6@pivotal.io>

cloudfoundry/php-buildpack,cloudfoundry/php-buildpack,cloudfoundry/php-buildpack,cloudfoundry/php-buildpack,cloudfoundry/php-buildpack,cloudfoundry/php-buildpack,cloudfoundry/php-buildpack