v0.4.0

• Made stoplights handle failing notifiers by logging the failure to standard
  error.
• Made stoplights automatically fall back to a fresh in-memory data store if the
  primary store is unavailable.
• Generalized Stoplight::Notifier::StandardError into
  Stoplight::Notifier::IO.
• Changed notification format from a string to a lambda. It accepts the same
  parameters that the format string accepted.
• Updated Stoplight::Notifier::Base#notify to accept three parameters (the
  light, the before color, and the after color) instead of just one parameter
  (the message).
• Prevented setting non-positive thresholds.
• Removed Stoplight::Mixin.