Fix pull request checkouts

For jobs which explicitly checkout a pull request branch, the reference
to the PR was not available, resulting in the following type of failure:

"""
git clone -b origin/pr/26/merge \
https://github.com/zerovm/validator.git /home/ubuntu/validator \
Cloning into '/home/ubuntu/validator'... \
warning: Remote branch origin/pr/26/merge not found in upstream \
origin, using HEAD instead
"""

This change properly configures the git clone ref specs so that PRs can
be checked out and tested.
Merge pull request #9 from larsbutler/fix-pr-checkouts

Fix pull request checkouts