Fix issue when not in a rails'y directory

the previous glob + finding the zeus socket file would hang when you
fired up vim in a directory that didn't contain that socket file (aka, a
lot) so this fixes it by:

1. testing the existence of a socket file named .zeus.sock
2. checking the exit code of that test command with v:shell_error
3. if it exited w/o an error, use zeus as the rspec command
4. if it exited w/an error, use rspec

Add a (some) shortcuts to toggle the rspec command if I want to.