Bump rspec to 3.6.0

• https://github.com/rspec/rspec-core/blob/master/Changelog.md
• https://github.com/rspec/rspec-expectations/blob/master/Changelog.md
• https://github.com/rspec/rspec-mocks/blob/master/Changelog.md
• https://github.com/rspec/rspec-rails/blob/master/Changelog.md