Fixes #799 - Adds tests for customs.js which hits the fxa-customs-server

IIRC, we should always use `--dev`. As always, @jrgm knows best and can explain the exact whys.

I started adding a `npm run shrinkwrap` script in the package.json, hoping that it'd make it easier to remember to add the `--dev` flag, but it's probably more awkward than not.