See https://travis-ci.com/github/NYPL-Simplified/server_core/builds/219120572 for an example.

Running the core tests involves pulling an image from Docker Hub. Running the tests both when a PR is created and when a branch is pushed can cause us to exceed Docker's rate limits, causing the test to fail.

I don't think we actually need to run both of these test suites–running the tests every time a commit is pushed should be sufficient. We can also resolve this by authenticating with Docker Hub to get a better rate limit.