Feature: update rdf gem to latest version

[syphax-bouazzouni]

Changes

• unpin rdf gem version to use the latest version
• add rdf/rdfxml gem to support rdf/xml serialization
• add rdf/vocab gem to no more include the rdf base gem (used for example to get SKOS namespace)

[codecov[bot]]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (27a3837) 85.90% compared to head (b298e65) 85.95%.

Files	Patch %	Lines
lib/goo/mixins/sparql_client.rb	70.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## development #56 +/- ## =============================================== + Coverage 85.90% 85.95% +0.04% =============================================== Files 41 41 Lines 2717 2705 -12 =============================================== - Hits 2334 2325 -9 + Misses 383 380 -3 ```

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