Add end to end testing during continuous integration of ezsystems/ezplatform-graphql ^0.4@beta.

While there is phpspec coverage for the schema generation, there is no end-to-end testing ensuring that basic features work as expected. It must be implemented, with actual queries (2 or 3 would do for now) against the major features. No preferences as for which tool it should use, as long as it runs on Travis.

Examples: query products, images, richtext conversion.