Create sketch files in PRs

Right now TravisCI runs tests, which also run the examples which generate Sketch files. It would be cool if we could actually download those Sketch files to make sure they are valid.

If we do that, we should also have more tests for each model that generate a Sketch file with different permutations of configurations for testing.

Start with the TravisCI configuration and the example-generation tests mentioned in the issue. Check how generated Sketch files can be downloaded and validated, then add coverage for model configuration permutations; done means CI verifies valid files and the models have the requested tests.

Written by the indexing model from the issue text.