Updating CircleCI config to be faster

[aendra-rininsland]

This does a few things:

1. Removes some of the weird caching behaviour Callum added to counter some of the bugs in early versions of CircleCI v2
2. Uses npm ci to install exact deps using package-lock.json
3. Removes the apt-get install step, instead using the Dockerfile here

As a result CircleCI builds should be much, much faster.

N.b., the Docker image I'm using is aendrew/starter-kit-docker:latest — I've namespaced it to my Docker/GitHub users because there's neither a Financial Times Dockerhub user nor one for ft-interactive. I figure I'll move the image and everything there eventually if and when those get created.

[aendra-rininsland]

Build time perf improvement:

• Baseline 224c4cdd399: 2:07
• 7cecbac 1:48 (-15% improvement)

I've moved npm cache to ~/.npm and made npm updating part of the base image:

• 40b9ee0 1:14 -- (-42% improvement)

[aendra-rininsland]

Let's not do this for the immediate future.