Validate license field against the SPDX identifier list

Uses a new 'spdx-licenses' gem for the list, and returns an error if this
recommendation isn't met.

The Puppet Forge uses the SPDX list to validate known licenses when scoring
the quality of metadata files.

https://github.com/puppetlabs/puppet-docs/pull/414 introduced this to the
module publishing documentation.
We could have warnings that print to stdout but dont fail the test. Then we could take a command line arg or eat a config file for fail-on-warnings. I personally prefer the command line arg because more dotfiles are not what any repo needs.