force checkout when deploying, to override any "local changes"

Sometimes the config repo replaces a file that is also edited in the main repo -- when this happens, deploys fail because git returns an error saying "local modifications would be overwritten, aborting".

This commit fixes this issue by adding the "--force" option to the "git checkout" command upon deploy. It would be dangerous if you cared about the "local" modifications on the remote box, but since this is designed to deploy to prodution systems, my opinion is that you should never be making important, uncommitted modifications on the production machine.

Feel free to disregard this pull request if you disagree with that opinion.