Can't get example to work

I'm on ubuntu 14.04 with the latest version of browserify.

I can't get the simple example to work

```
git clone git@github.com:substack/factor-bundle.git
cd factor-bundle/example/files
mkdir bundle
browserify x.js y.js > bundle.js
browser-unpack < bundle.js | factor-bundle x.js -o bundle/x.js y.js -o bundle/y.js > bundle/common.js
```

This results in the creation of `bundle/common.js` but no `x.js` or `y.js`

Reproduce the issue in example/files on Ubuntu using the listed browserify, browser-unpack, and factor-bundle commands. Inspect the generated bundle.js and bundle/common.js, then trace why bundle/x.js and bundle/y.js are not created. Done means the example produces all three expected output files or clearly documents the supported behavior.

Written by the indexing model from the issue text.