fix bundle install windows

shelling out on windows with backticks or `system()` does not yield a unix type shell. e.g.:

```
$ ruby -e 'puts `A=B; echo $A`'
-e:1:in ``': No such file or directory - A=B; echo $A (Errno::ENOENT)
	from -e:1:in `<main>'
```

This patch works for windows as well as unix as far as I can tell