Remove list of modules from README

This is now out of step with reality. Give a brief summary instead.
Remove specs of Vagrant VM from README

Because there's a good chance these will get out of step with reality.
The `Vagrantfile` should act as living documentation for these things,
including what providers work.
Simplify testing instructions in README

It's not always best practice to destroy the box between every change
because you're no longer testing convergence, from the current state in
production to the new state, which is often where unexpected bugs lie.
Merge pull request #83 from alphagov/update_readme

Update README