Ensure 'wait_for_completion' does not exit early

Thanks for the great gem!

This PR ensures that `wait_for_completion` accounts for more non-terminal Amazon EMR states, [documented here](http://docs.aws.amazon.com/ElasticMapReduce/latest/DeveloperGuide/ProcessingCycle.html).

I noticed this bug while I was running 3 streaming map reduce steps with `elasticity`, and the `wait_for_completion` loop was exiting on the `BOOTSTRAPPING` action, way before the job finished.