Add support for Puppet 4

[ghost]

class inhouse_airflow::install {
  class { 'airflow': }
}

Error: Could not retrieve catalog from remote server: Error 500 on SERVER: Server Error: Evaluation Error: Error while evaluating a Function Call, validate_integer(): Expected first argument to be an Integer or Array, got NilClass at .../airflow/manifests/init.pp:219:3 on node node.example.com

puppet version - 4.10.5 airflow module version - 0.1.2 kernel - Linux 3.10.0-514.el7.x86_64 system - CentOS Linux release 7.3.1611 (Core)

I understand Puppet 4 is not outright supported, but it would be great if this could be done