A better fix for #357.

This is a better fix for several reasons:

- It's more portable; the arguments/options to 'ps' vary widely across
  platforms.
- Killing children by forking yet more children (i.e., via backtick) is
  problematic when you're trying to kill processes because of resource
  starvation.
- Processes should not signal processes other than their own children,
  especially in a generic task-runner like Foreman. If the signal
  should propagate, then the sub-process should propagate signals to its
  children itself. There's no way to know what cleanup or preparation
  is necessary for a clean shutdown (e.g., SIGINT/SIGTERM), and
  in what order grandchild processes should be shut down to properly
  release locks, close files, etc.
- foreman-runner doesn't (shouldn't?) create grandchild processes; the
  last thing it does is `exec` the program it's launching, which
  will replace the program of the *current* process with the one
  being started up.