Fetch artefact for slimmer headers
Simplify feature setup
Simplify artefact retrieval
Simplify artefact format headers
Set slimmer format to 'calculator'

This also fixes the error when the artefact is nil.

Also cleaned up the test setup.
Merge pull request #33 from alphagov/fetch_artefact_for_slimmer

Fetch artefact for slimmer