This shouldn't be part of the PR, but I noticed that the wrong argument is being passed into phantom-cluster. It should be called `workerIterations` rather than `iterations`.

Upped the default from 10 to 100. Since this argument was being ignored by phantom-cluster, it was using its own default to 100. By changing the default here, we're ensuring behavior of existing prerender installations doesn't change unless they've explicitly set `PHANTOM_WORKER_ITERATIONS`.