Figure out CircleCI caching to speed builds

[jonrh]

At the time of writing I have successfully set up CircleCI to automatically deploy static files to S3 on AWS and to invalidate the CloudFront cache. The build takes about 2 minutes but could be improved in a few ways.

1. First I'm manually installing awscli with a bunch of overhead. It may be that CircleCI is clever enough to use that as a cache but maybe not.

2. The big gain is going to be in node_modules/. I tried to cache on the yarn.lock file but it didn't seem to work (see commit 8a58059). In related commits I had it set up for package.json. Maybe that's good enough.

Here is an example config:
https://github.com/CircleCI-Public/circleci-demo-javascript-express/blob/master/.circleci/config.yml

Overall I'm not too concerned about this at the moment, 2 minutes is fine.

[jonrh]

Website is now automatically deployed with Vercel so there is no need for CircleCI anymore. See #29.