instruments process are becoming orphaned because the parent is killed before the child

### Summary

Instruments process continue to be orphaned.  Previous fixes (by me) have been incomplete.

We need to do two things:

1. Use 'command' vs. 'comm' to get both the parent and the child instruments processes.
2. The child process should be killed first and then the parent.

This PR reverts (pre Xcode 6) behaviors implemented by @krukow with some additional guarantees that the child instruments process is killed before the parent instruments process.

### Acceptance

1. No orphaned instruments processes; we will only know if we have succeeded after long-term testing.
2. Should see logging statements like this:

```
$ DEBUG_UNIX_CALLS=1 be rspec spec/integration/instruments_spec.rb
Sending 'QUIT' to instruments process '69628'
Waiting for instruments '69628' to terminate
Sending 'QUIT' to instruments process '69629'
Waiting for instruments '69629' to terminate
```