Fail if environment is not healthy.

Environment#healthy? checks that the Beanstalk status is green.
This is used in the beanstalkify script to raise an error rather than cheerfully exitcode 0'ing when the new environment has really failed to launch.
Display CNAME.elasticbeanstalk.com rather than ELB URL.

This is probably more use to people who want to refer to the environment in the future.
Merge pull request #6 from rickgrundy/master

Fail if environment is not healthy.