metadata.json: bump allowed version of puppetlabs-apt to 6.0.0
metadata.json: bump allowed version of puppetlabs-apt to 6.0.0

Changelog: https://forge.puppet.com/puppetlabs/apt/changelog#500-2018-07-18

This is to avoid warnings with dependencies.

DavidS/puppetlabs-postgresql,puppetlabs/puppetlabs-postgresql,buzzdeee/puppetlabs-postgresql,infoxchange/puppetlabs-postgresql,npwalker/puppetlabs-postgresql,npwalker/puppetlabs-postgresql,buzzdeee/puppetlabs-postgresql,DavidS/puppetlabs-postgresql,npwalker/puppetlabs-postgresql,tdevelioglu/puppetlabs-postgresql,puppetlabs/puppetlabs-postgresql,DavidS/puppetlabs-postgresql,infoxchange/puppetlabs-postgresql,infoxchange/puppetlabs-postgresql,puppetlabs/puppetlabs-postgresql,tdevelioglu/puppetlabs-postgresql,duritong/puppetlabs-postgresql,buzzdeee/puppetlabs-postgresql,tdevelioglu/puppetlabs-postgresql