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.
It's the base commit of the pull request. The head commit would be the merge commit SHA (which is hidden on GitHub until you hit the merge button).