Fix check for commit ranges in racing PRs.

See #1768 / #2000 for original issue.
See https://github.com/travis-ci/travis-ci/issues/1719#issuecomment-51705110 for problem description and origin of this fix.
This is still a temporary fix until travis-ci/travis-core#383 is merged.
Merge pull request #2152 from RReverser/fix-pr-check

Fix check for commit ranges in racing PRs.