Add 'ruby-graphviz' gem to development_dependency
Add 'ruby-graphviz' gem to development_dependency

When `bundle exec rake` to run tests locally, ruby-graphviz is needed.
Sure `bundle exec rake test:all` ensures the gem is installed for
all rails versions, but `bundle exec rake test` (the default task) fails
without 'ruby-graphviz'.

amatsuda/stateful_enum,amatsuda/stateful_enum