Update license in metadata.json

I noticed "Unrecognized license in metadata" in https://forge.puppetlabs.com/dwerder/redis/scores .

The identifier this -- https://docs.puppetlabs.com/puppet/latest/reference/modules_publishing.html -- looks to match seems to be the identifier column (rather than name) here -- http://spdx.org/licenses/ -- which for some reason is called a short_identifier here -- http://spdx.org/licenses/MPL-2.0 .