Fix infinite join bug when calling `stop` on a Service

This fork fixes the bug where calling `stop` on the service returned from methods like `DNSSD.browse` would enter an infinite `join` call by trying to `join` the current thread, despite a condition in `service.c` not to. I just commented out the line (160) in `service.rb`:
```
@thread = Thread.current
```
Although that's supposed to avoid the `join` block, it doesn't. Leaving `@thread` as `nil` avoids the block, and has no effects (I can't find anywhere besides that block in `service.c` where `@thread` is actually used).

See issue #10