pre-upgrade path should be relative to cwd

If I have liveswap installed globally and execute the following:

```bash
$ liveswap -s script.js --pre-upgrade ./pull.js
```

It will look for `./pull.js` relative to the location of the liveswap module. It seems to me to make much more sense to make the location relative to `process.cwd()` when executed from the command prompt. This change is made in the bin script so as not to impact usage as a module.