(maint) Update puppet-agent min required version to 5.5.0
(maint) Update puppet-agent min required version to 5.5.0

Puppet Server 6 needs to be installed with an agent >= 5.5.0, which
provides the shared gem dir that it relies upon for gems shared between
the agent and the JRuby server.

puppetlabs/puppetserver,puppetlabs/puppetserver,puppetlabs/puppetserver,puppetlabs/puppet-server,puppetlabs/puppet-server,puppetlabs/puppetserver,puppetlabs/puppet-server