Allow `puppetlabs/stdlib` 6.x
Allow `puppetlabs/stdlib` 6.x

stdlib 6.0.0 is due to be released soon.
This module has already dropped support for puppet 4 and/or doesn't
use the updated stdlib `merge` function.

See
https://github.com/puppetlabs/puppetlabs-stdlib/blob/217068f97edd2396ce37ac8c1bd7e23da621d6be/CHANGELOG.md#supported-release-600

puppet-community/puppet-extlib,voxpupuli/puppet-extlib,duritong/puppet-extlib