Do a shallow clone of the everypolitician-data repo

[chrismytton]

Problem

The build process is very slow currently because we do a full clone of the ep-data repo.

Proposed solution

Do a shallow clone with --depth=1. This will mean changing how countries.json gets built, because that currently relies on the full git history. But we can just disable the building of countries.json for now and/or change how it works to fit with this.

Prerequisites

Moving the build steps to a shell script, #67, will make doing this much easier.

Related issues

Part of the bigger refactoring being tracked in #62.