Improve VM shutdown for Sauce Labs connector

- Add retries for VM shutdown when talking to the API.
- Add 'ensure' to make sure the SSH connection is not reused.
Make sure we actually have a clean VM

If a VM is somehow being reused without clean data on it, we can detect
this easily by requeueing if a build.sh file already exists.
Fix conditional for build.sh existence check

If the script returns 0, the file exists, so we should error.
Add note in README about issue tracker location
update travis build
Fix error message for Sauce Labs VM boot errors
Update travis-build
Update the boot sequence for Sauce Labs VMs
update travis-build
Merge branch 'bluebox-production-org' of github.com:travis-ci/travis-worker into bluebox-production-org