Using #simple? instead of #plain? for literals without language

RDF::Literal#plain? allows a language tag for the literal, so `unserialize_localized_properties` should use RDF::Literal#simple? instead.

@abrisse could you take a look and confirm that everything's as you originally intended?
Apparently, you're not using RDF.rb which is "bundled" for development (1.0.6 here), so things might be different there.

That said, I vote for getting `Gemfile.lock` back into the repository, @gkellogg ;-)