Fix flow issue in lib/Configuration.js
Fix flow issue in lib/Configuration.js

I had mistakenly declared Configuration.loadUserConfig() as always
returning an object. Which is not true, sometimes `undefined` is
returned.

This passed the Travis build on the `javascript-configuration` branch
because we don't treat flow issues as errors yet. I'm about to fix that
soon.

Galooshi/import-js,trotzig/import-js,trotzig/import-js,trotzig/import-js