reviewboard update

[Eric Snow]

FYI, I was able to solve 3 reviewboard-github integration issues today:

1. pull requests for branches other than master now work (e.g. 1.21 backports)
2. no more hitting rate limits (5000 requests/hour limit instead of 60)
3. pull request bodies now get updated with a link to the new review request

If you have any trouble with any of these please let me know.  Thanks!

-eric