Upgrade rdf gem (RDF.rb library) to a more recent version

[jvendetti]

In October, 2013, the 'rdf' dependency was pinned to version 1.0.8. This is a very old version of the gem, released 7 years ago in September, 2013. There's no documentation in the commit, indicating why RDF.rb was pinned at 1.0.8.

This issue was precipitated by the desire to merge upstream changes into our fork of the sparql-client gem. The upstream code uses version 3.1.3 of RDF.rb, which is incompatible with version 1.0.8 used here in goo.

[syphax-bouazzouni]

For reference to the work done on this at AgroPortal ontoportal-lirmm#56