Release 2.2.0

• Updates for RDF.rb 2.2.
• Update minumum ruby version to 2.2.2. See ruby-rdf/rdf#307.
• Tests Transaction#mutate?
  • We deliberately avoid checking for false positives, since the interface makes no guarantee that they won't appear.
• Generalize protected method check for Queryable
• Remove false as default assumption in Queryable specs
  • This assumption has only held for RDF::Repository in the past. The test should be replaced in either Repository or Dataset.
• Remove duplicate Transactable tests from Repository
• Isolate Transactable#transaction test from blank nodes
  *This test used to implictly test stablity of blank node identifers across multiple #statements calls. By using URIs instead of blank nodes, we avoid failures when they change.
• Filter for #supports?(:literal_equality)
• Use #to_h instead of #to_hash. Test for DEPRECATION on uses of #to_hash. This is associated with ruby-rdf/rdf#333.