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

Yes and no. Yes, it should probably follow the ones around it (so I'll fix it).

No because, from what I can tell, the devDependencies are not saved in the `npm-shrinkwrap.json` file. I'd love confirmation from someone else if they also see this. I found it because, after I'd added this line I regenerated shrinkwrap and nock was nowhere to be found. I'll double check but I think this is the case.