update deployer/deployer to version 7.

The prior merge of !1743 (merged) did fail when deploying, as deployer/deployer needs to be in version 7 in order to find the moved recipes in the contrib folder.

This update tries to fix that.

The new way of loading the cache-tool as described in https://github.com/deployphp/deployer/blob/master/contrib/cachetool.php#L6 is only available with version 7 as they have been moved into this repository after the release of version 6.8 used by us.