Fix compatibility with the pre-4.0.6 Review Board API.
Fix compatibility with the pre-4.0.6 Review Board API.

Review Board 4.0.6 introduced the `binary` flag on filediffs, but that
wasn't present on earlier versions. Recent code to Review Bot made this
flag a requirement. This change fixes it to conditionally check for its
presence instead.

This is a trivial change.

reviewboard/ReviewBot,reviewboard/ReviewBot,reviewboard/ReviewBot,reviewboard/ReviewBot