Set up build checking in CI

[aaronsarnat]

It would be nice to be able to catch code that would cause a build failure before a PR gets merged.

Note that there may be a special way to handle this that ties in with Skiff and Google Cloud Build, so it would be advised to check w/ Sam or Jon before setting this up.

[aaronsarnat]

Also, looks like Marina isn't displaying build statuses:

https://marina.apps.allenai.org/a/allennlp-course

See https://github.com/allenai/skiff/blob/master/doc/UserGuide.md#why-doesnt-my-application-have-build-statuses-version-information-and-adhoc-support