Foreman does not terminate child process

Same issue as https://github.com/ddollar/foreman/issues/628

Procfile:
```
worker: ruby test.rb
```

test.rb
```
pid = Process.spawn("sleep 100")
Process.detach(pid)
sleep
```

Steps to repoduce:
1. `$ foreman start`
2. `$ kill <foreman-pid>`
3. Spawned process is still running