When puppet7 becomes available and the linting check installs puppet7, it will fail as Puppet7 only supports ruby 2.5+

I have tested the template change locally and can confirm it only changes the Ruby version for appveyor. I have also put up a PR with the change to confirm it works as expected. PR can be found here: https://github.com/puppetlabs/puppetlabs-acl/pull/223