Returns max number of threads machine can handle with a min of 1

Keys in the remains hash represent uninstalled gems specs. We enqueue all gem specs that do not have any dependencies. Later we call this lambda again to install specs that depended on previously installed specifications. We continue until all specs are installed.

Dequeue a spec and save its post-install message and then enqueue the remaining specs. Some specs might‘ve had to wait til this spec was installed to be processed so the call to `enqueue_specs` is important after every dequeue.