Since GitHub 2.21 we now can query the reviewDecision flag on a PR
which tells us if a review is required and if it's approved or not
[1]. This can be leveraged in the canMerge check so we now finally can
accurately check if a change is allowed to enter the gate.
[1] https://developer.github.com/enterprise/2.21/v4/object/pullrequest/
Change-Id: I792a28b9f3c7d40ac21e22438bd7c09d3174cbb2