Install phantomjs on the frontend machines

It's required for the new screenshot-as-a-service app, which is a Node
thing that returns PNGs. We're going to use it for Spotlight fallbacks.
Add screenshot-as-a-service application

Creates screenshots that can be used by Spotlight for fallback images.

eg Spotlight can call http://screenshot_as_a_service/?url=http://localhost:3057/dashboard-url
screenshot-as-a-service always needs NODE_ENV prod

We want to load the production.yaml config file no matter which
environment we're in. This app only has support for one config
file in a production-like environment at the moment.
Merge pull request #136 from alphagov/add-screenshot-service-app

Add screenshot-as-a-service app