How to configure Travis-CI to build pull requests & merges to master w/o redundancy

I eventually found another GH issue (#2111) which gave me the idea to try enabling both PRs & pushes, but with a whitelist to restrict pushes to a specific branch. This seems to satisfy the criteria for my workflow. Here’s what I did:

1. Enable both PRs & branch pushes in the Travis settings for the repo:

2. Change .travis.yml to white-list master branch (i.e. only build pushes to master):

branches:
  only: 
    - master

3. Test it by creating a PR with the .travis.yml change, and another PR with some empty commits to verify it works for forks too.

4. Verify successful merge commit build from master.