Enable running rake through zeus if it is present and running

I don't know how useful this is for anyone else, but I use [Zeus](https://github.com/burke/zeus) with my Rails apps. This will allow Rake to run much faster if [Zeus](https://github.com/burke/zeus) is started. I check both for the existence of `.zeus.sock` and a running process because if the machine crashes `.zeus.sock` will still exist, even though [Zeus](https://github.com/burke/zeus) isn't running, and it's not useful to check for a running process if the file doesn't exist at all.