Force link behaves differently than on OS X

### Node version
9.2.1
### ShellJS version (the most recent version/Github branch you see the bug on):
0.8.1
### Operating system:
OS X
### Description of the bug:
On OS X, `ln -sf ....` works even if the existing link is invalid. With `shelljs`, which uses unlink under the hood, the `ln -sf` doesn't work if the existing link isn't valid.

I'd suggest replacing unlink with `rm`.
