Rakefile: make validate work with both Puppet 3.x and 4.x
Rakefile: make validate work with both Puppet 3.x and 4.x

SoftwareHeritage/puppet-swh-site,SoftwareHeritage/puppet-swh-site,SoftwareHeritage/puppet-swh-site,SoftwareHeritage/puppet-swh-site,SoftwareHeritage/puppet-swh-site