Refactor Launch to validate in .new, not .run.

Better to do set up and validation when the class is loaded, rather than defer it to when we want to get work done.
Remove unnecessary comment following PR review by @mattbostock.
Use @config instance variable directly, following PR review.
Ensure Cli.run calls Launch.run following PR review.

I'm avoiding validating the instantiation of Launch as that's handled in earlier tests, wrapped with the CommandRun class in the spec.
Merge pull request #67 from bazbremner/refactor_launch_validate_during_initialization

Refactor Launch to validate in .new, not .run.