Return the right commit range for pull requests

The compare URL for pull requests is just the pull request URL, so we can't extract the commit range from it, but the commit range is stored in the request for pull requests, so we can get it from there.

Fixes travis-ci/travis-ci#1719.
Ok, thanks for the answer! Can you estimate when this change will go live? I would love to start using `$TRAVIS_COMMIT_RANGE` in my pull requests