do not run in background shell thread

[MaxPleaner]

This gem right now is totally asynchronous with no good facility to halt until the downloading is done. This does not have to be the case because the youtube-dl shell program is blocking. It would be blocking if the & were removed in &/dev/null, I think. I suppose I would have to find another way to hide the output, but it isn't hiding the output right now anyway.

It seems obvious that this gem should be able to be used in a blocking manner from a typical ruby script, but I actually didn't consider this when I was writing it, because my use-case was for using it from a music-creation REPL (step_sequencer).