Change to staticfile buildpack

[MatMoore]

We're using the ruby buildpack when deploying to PaaS, but we don't actually need it.

There is a static files one we can use instead - example here: https://github.com/alphagov/registers-tech-docs/blob/master/manifest.yml

[MatMoore]

I think the steps here are:

• Change ruby_buildback to staticfile_buildpack in the manifest.yaml
• Make sure https://github.com/alphagov/tech-docs-template/blob/master/optional/nginx.conf is generated for this repo. When you run the gds-tech-docs-gem from scratch, this is done if you for you if you run the optional "I'm deploying to the paas" step.