Better handling for .zeus.sock

Features:

 - An expired .zeus.sock (that cannot be connected to) will be deleted
 - A current .zeus.sock (that can be connected to) will skip `zeus start`
Bit of a stylistic comment but I'd change order of statements:

```
before do
  allow(subject).to receive(:sockfile).and_return(sockfile)
  allow(File).to receive(:exist?).with(sockfile).and_return(true)
  allow(UNIXSocket).to receive(:open).with(sockfile).and_return(socket)
end
```