v4.0.0

• Updated for the Node.js 4.3 runtime.
• Refactored steps to use promises.

Upgrade Notes

• The Node.js 4.3 Lambda runtime is required for this version.
• The arguments for the handler function have changed to match the Lambda
  runtime for Node.js 4.3 to include a callback function. If invoking the
  handler function, be sure to include the callback function provided in the
  Lambda runtime environment.
• Step functions that include asynchronous operations should return a promise,
  and are no longer provided a callback function as an argument. Upon successful
  completion of the step function, call Promise.resolve() with the data
  argument given to the step function. (Step functions that are synchronous should
  return the data argument.)