https://github.com/bbatsov/rubocop FTW. Integrate it into your test suite and you can see your mistakes before you push up a change for your team to see.
We've tried putting style violations in the build and found there are enough edge cases that it isn't exactly the interaction we've wanted. Sometimes, we want the human to say "no, my pair and I broke the guideline on purpose and we're okay with it in this case." We don't want a broken build in those cases.
Totally agree with you about getting feedback earlier than opening a PR, though. Linters integrated into text editors are a great way to go.
Rubocop can even correct some style errors automatically now!