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, that's what I thought. FWIW, this is not what your current documentation claims for `$TRAVIS_COMMIT_RANGE` e.g. `The range of commits that were included in the push or pull request`. However, I personally prefer this approach, so please don't change it!! :-)