github statuses feature doesn't count correctly

[metajack]

When using the status API, bors doesn't do any counting of successes. If the number of successes is greater than zero and it sees no failures, it will assume it passed.

For some reason, some status updates aren't going through, so bors will see successes=1 and failures=0 and not wait for the second builder's status to show up. This caused it to land a few PRs that failed tests in Servo.

[metajack]

cc @reiddraper