Fix clippy::zombie_processes warning

```
error: spawned process is never `wait()`ed on
  --> src/reaper/signal.rs:66:25
   |
66 |                         zombies.swap_remove(i);
   |                         ^^^^^^^^^^^^^^^^^^^^^^- help: try: `.wait()`
   |
   = note: not doing so might leave behind zombie processes
   = note: see https://doc.rust-lang.org/stable/std/process/struct.Child.html#warning
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#zombie_processes
   = note: `-D clippy::zombie-processes` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(clippy::zombie_processes)]`
```

Author: taiki-e

src/reaper/signal.rs

@@ -63,6 +63,7 @@ impl Reaper {
                     if let Ok(None) = zombies[i].try_wait() {
                         i += 1;
                     } else {
+                        #[allow(clippy::zombie_processes)] // removed only when process done or errored
                         zombies.swap_remove(i);
                     }
                 }