Fix continuous deployment of Frontend Docs

[36degrees]

What

Fix continuous deployment of the GOV.UK Frontend docs, which is currently broken.

Why

The deploy stage is failing on the master branch, which means changes are no longer being deployed automatically.

Starting deployment for app govuk-frontend-docs in org govuk-design-system / space frontend-docs as design-system-deploy+frontend-docs@digital.cabinet-office.gov.uk...
OK
Waiting for app to start...
Start app timeout
TIP: Application must be listening on the right port. Instead of hard coding the port, use the $PORT environment variable.
Use 'cf logs govuk-frontend-docs --recent' for more information
FAILED
Script failed with status 1

We think this is because we're still using the v3 prefixed commands to deploy:

govuk-frontend-docs/bin/deploy-travis

Lines 11 to 14 in a7f28e4

	cf v3-create-app $APP_NAME
	cf v3-apply-manifest -f manifest.yml
	# Deploy using CloudFoundry zero-downtime push
	cf v3-zdt-push $APP_NAME -p $APP_PATH --wait-for-deploy-complete

And, roughly:

• v3- commands were experimental, while they worked on the API
• v7 CLI came out, and uses the v3 API in a not-experimental way (i.e. it’s supported properly)
• The v3- commands weren’t updated to work with newer versions of the API

So we think the solution is to upgrade to the V7 version of the CLI, and use non-prefixed commands.

Further details

The same issue came up in the learn-to-code repo, and was fixed by the changes in alphagov/learn-to-code#40.

It might make sense to do #89 at the same time.