Automatically execute dummy task to precompile dependencies

[Curtis Vogt]

At the moment if attempt to start a bunch of workers where not all of the Julia packages have been precompiled then each worker will attempt to precompile the packages. These processes can interfere with each other so it's usually more sensible to run a warm up task like:

Ray.submit_task(getpid, ())

We should perform this operation within Ray.init for now until we get precompilation worked out.