change default reference_point to HEAD (was master)

Create from https://github.com/mattpolito/paratrooper/pull/64. There is some initial discussion on that PR. This PR isolates the issue.

This makes sense to be as the default. If you want a set branch, simply set it with `branch: 'master'`. Obviously this change is backward incompatible so it may require a major version release.

If it is decided that the default should remain to master, I'll force push an update that keeps master as the default but allow for the config option of `branch = :head`. Let me know.