r=me post-travis feature

[Manishearth]

Rust runs almost the entire testsuite on travis, and because the queue is slow it's usually preferably to do an "r+ post travis"

It would be nice if we could make this part of Homu, where we list status contexts in the homu config, and you call homu r+ when-green, where homu puts it into the final queue only when the PR travis is green (and complains when it isn't)

Ideally this could also include an autorollup feature, every N travis-green PRs get automatically rolled up, unless marked otherwise (some kinds of PRs are more prone to breaking the build)