convert Process.fork call to Process.spawn

``Process.fork`` makes ``rmate`` tricky on Windows environments, and not using windows is a great use case for ``rmate``.

The downside is that ``fork`` is replaced by ``spawn`` (and a bit of ``RbConfig``), so the changes in the pull request require ruby 1.9.x.  If the version requirement is a dealbreaker, I can clean the gem parts out of this repo, and maintain is as a port like the bash script port (which doesn't solve this case -- if an environment supported ``/dev/tcp``, it probably supports ``Process.fork``).