Provide a gemspec licenses section

[hlascelles]

This gem does not have a licenses in its gemspec: https://github.com/w3c-validators/w3c_validators/blob/b227ae81a861fbdb92dbe04c795979badb0fab19/w3c_validators.gemspec

This would be good to have for organisations that need to verify the licences of the tools they use!

I can see the license file, but we track licences automatically driven from the gemspec using this: https://github.com/pivotal/LicenseFinder