return empty array instead of undefined in Packer.prototype.readRules()

When trying to install browserify 0.4.5 to 1.0.0 npm crashes with the following message:

```sh
TypeError: Cannot call method 'filter' of undefined
    at Packer.IgnoreReader.addIgnoreRules (/home/magnus/src/fstream-ignore/ignore.js:148:13)
    at Packer.IgnoreReader.addIgnoreFile (/home/magnus/src/fstream-ignore/ignore.js:133:10)
    at fs.readFile (fs.js:176:14)
    at Object.oncomplete (fs.js:297:15)
```

This happens because JSON.parse() fails in Packer.prototype.readRules() which returns undefined which in turn causes IgnoreReader.prototype.addIgnoreRules() to blow up.