To compare PullRequestReview objects, they need to have the 'url'
attribute because the CompletableGithubObject expects to be able to
inspect a '_url' attribute on objects when testing for equality.
This field is not currently listed in the Github documentation at
https://developer.github.com/v3/pulls/reviews/, but it's been included
since Github 2.9
(It even exists in the test data returned for the original
PullRequestReview PR)