Update README to mention govuk-provisioning

We're no longer using vcloud-templates for node definitions as of
alphagov/vagrant-govuk@530ff06da4142f7739dce2d64fa6cbd74eaac7bd.
Complete 'prereq' to 'prerequisites'
Change JSON to YAML for local customisations

Since the change to use the alphagov/govuk-provisioning repo for node
definitions, we now use YAML for local node customisations.

Note that although YAML is a superset of JSON, vagrant-govuk will only
accept a file ending in a JSON extension[1].

https://github.com/alphagov/vagrant-govuk/commit/530ff06da4142f7739dce2d64fa6cbd74eaac7bd#diff-ca613347e8e86c8679ba2d0383c56cfcR14
We no longer use Lucid on GOV.UK; remove mention
Merge pull request #27 from alphagov/update-readme

Tidy README