Release 0.3.0

• Minimum Ruby version now 2.6.
• GitHub testing with virtuoso service (#16) (@stuzart)
  • include virtuoso as a service in CI
    • include the tenforce/virutoso docker container as a service in the github ci.yml
      however, currently just the service, not yet included in any tests #15
  • a basic integration test that does a simple query #15
    • a simple check that the repo is connecting and querying within the Github CI environment
  • the default graph is unlikely to be needed #15
  • rubocop on integration spec
  • extend tests to check the connection is working and some cleaning up
  • check with a single select which is unlikely to change, and also include a simple insert #15
  • fixed invoking it_behaves_like but commented out until resolved
  • explicitly setting the let(:repository) caused an infinite loop and stack overflow, because the subject is already named it works without #15
  • control whether the integration tests are run with an env variable #15