Change travis cleanup to skip_cleanup

[sshanks-kx]

skip_cleanup was previously giving a config warning - recommended to change to cleanup

For release on another repo it printed
"Cleaning up git repository with git stash --all. If you need build artifacts for deployment, set deploy.skip_cleanup: true. See https://docs.travis-ci.com/user/deployment#Uploading-Files-and-skip_cleanup."
and caused release to fail.

Changing back now gives no warning