Limit ci:list to a specific branch

[gulopine]

We recently switched to Heroku CI, and we've been using successful CI build output on the master as a prerequisite for a production deploy. We can use heroku ci or heroku ci:list to get a list of recent build results, and grep that list for master and check whether the latest is successful, but that only works if master happens to be in the last 15 test runs. If we've had a lot of development activity without a recent merge, it's entirely possible that we won't have a master build result in the list at all.

It'd be ideal if we could supply a branch name to the command, perhaps --branch master, and retrieve the latest results for only that branch. That way, we can be sure to have something valid to check before deploying.