marks the slowest specs with :slow

• use bundle exec rspec --tag '~slow' to skip slow tests
• use bundle exec rspec --profile to see slowest tests
• makes it possible to run the bulk of the test suite in roughly 1 second as opposed to roughly 5 seconds
• namely ./spec/acceptance/sequence_setting_spec.rb:309 alone was taking up 61% of the time to run the entire suite (3.12 seconds out of the 5.09 seconds). It seems like a useful test to retain, but is not critical to run while developing other features.