Run schema tests against govuk-content-schemas repo

Changes the tests that validate publishing schemas so that they reference the external alphagov/govuk-content-schemas repository. The location of the local checkout of this repo can be overridden via an environment variable, but it defaults to a sibling directory of Whitehall. The Jenkins script is updated to automatically check it out to /tmp before running tests.

Additionally a rake task `test:publishing_schemas` is provided to run just the publishing presenter tests, for use in the branch build of govuk-content-schemas.
Do you think it's worth failing earlier than this? i.e. should there be a check in `test_helper.rb` so that any attempt to run tests will fail unless govuk-content-schemas is present? Fail fast, fail early and all that...

I also think the error messaging could be more helpful - For a start, the ENV variable is only required if govuk-content-schemas isn't in the default location.How about separate errors for govuk-content-schemas not being present (the early check) and a specific one for when the schema in question cannot be found. e.g.

"Dependency govuk-content-schemas cannot be found. Ensure it is checked out in the same parent directory as the whitehall application (see README.md for more details)"

and

"Schema file not found: #{@schema_path}. Make sure it is present in govuk-content-schemas "