Improve README

- Mention the development VM in the intro
- Remove duplication in Ruby setup
- Remove Backdrop installation instructions (that's not how we do it now)
- Update to reflect files moving
- Remove Ruby warnings (we don't use Puppet 2.7 anymore)
- Remove things that are not relevant to the development directory into a
  more appropriate place
Lint modules

Mostly whitespace changes.
MIT license all the things
Merge pull request #434 from alphagov/improve-readme

Improve README