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.
@nibalizer true on the dot files point, though it makes it harder to either use or disable this feature if you're using the default puppetlabs_spec_helper rake task that I added.  I could add a `--fail-on-warnings` argument here, then enable it by default in pl_spec_helper, then let people redefine the task if they don't want it.

I think what you/we decide here would be applicable to a bunch of other "high quality"/"should" requirements for metadata files.