Travis/AppVeyor may miss a webhook

[alexcrichton]

Occasionally (likely due to the nature of webhooks) either Travis or AppVeyor may not actually start a build once a merge commit is created on the auto branch. For example with rust-lang/rust#39518 when homu started a build it scheduled a build on Travis but not AppVeyor (configured for the repo at this time).

It'd be awesome if homu would automatically detect this and retry the build if AppVeyor/Travis don't come back with a "build started" in a grace window of something like 5 minutes.