Bumps [rdf](https://github.com/ruby-rdf/rdf) from 3.1.5 to 3.1.6.
- [Release notes](https://github.com/ruby-rdf/rdf/releases)
- [Changelog](https://github.com/ruby-rdf/rdf/blob/develop/CHANGES.md)
- [Commits](https://github.com/ruby-rdf/rdf/compare/3.1.5...3.1.6

)

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