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.
Oh, wait. I see now, it is kind of consistent with the documentation, but just in a confusing way. That sentence makes me think that if I put in a PR with commits A, B, C, then push a few more commits D, E, then my `Travis_commit_range` for the first push is `A...C` and for the second push is `D...E`. 

You're saying that with this fix, my first push range will be `A...C` and the second will be `A...E`