Jobs record the commit underneath an annotated tag.

Currently when jobs are executed for an annotated tag, the job
records the sha of the annotated tag, not the commit it points to.

This will look up the commit that the annotated tag points to and
store that sha instead.
Merge pull request #24 from zendesk/josh/compute_proper_commit_from_annotated_tags

Jobs record the commit underneath an annotated tag.