Do not require aio_agent_version to be present

[fizmat]

This was started in PR #41 but one more check was left behind in device_manager::devices

I don't even think it's an enterprise/non-enterprise distinction. It looks like packaging differences:
When facter is built as part of the puppet-agent package, as it does in the puppetlabs yum repo, it gets the agent version fact compiled in from the config variable. Hence, I suspect, the aio (all-in-one) prefix.
If facter lives in a separate package, as it does in Arch official repos, it can't have the fact compiled in, as the agent version can change independently. At least this is the way I understand it.

As an Arch user, this is the second puppet module where I run into problems with missing aio_agent_version.

[DavidS]

As an academic, it is my opinion that it's Arch's responsibility to provide a runtime environment suitable for existing content to work, if they diverge from the official packaging.

As an engineer, I agree that the check is overzealous.

Thanks for your contribution!