Refactor schema examples to make them accessible

Now the schema examples can be used in model and controller tests as well.
Remove redundant fixtures

The tests now use the examples from govuk-content-schemas
Wire in rake task for running contract tests

This both adds a test:contracts rake task and wires it into the default test run
so that contract tests are executed as part of the default rake task.
Raise exception if can't find govuk-content-schemas

This is a bit more user friendly then the tests blowing up with a cryptic
message.