Shellwords is for posix path escaping, we shouldn't use it on windows

See #1718 for background.
I think we need to require `rbconfig` if we're going to use it here.  On some rubies, it's not automatically loaded:

```bash
➜ ruby -v
ruby 1.8.7 (2013-06-27 patchlevel 374) [i686-darwin12.4.0]
➜ ruby -e "puts RbConfig.name"
-e:1: uninitialized constant RbConfig (NameError)
➜ ruby -r rbconfig -e "puts RbConfig.name"
Config
```

It's probably working because something upstream is requiring it (e.g. bundler) but we can't count on that for end users.


It would also be good to confirm that we are loading `rbconfig` in the other places that use it.