travis: squash check

This checks at the very end of the build if a pull request needs squashing, based on the fact if there are any commits that start with a "SQUASH" tag between `HEAD` and `RIOT/master`.
I think, that - especially after squashing - waiting for Travis is mandatory. But you're still correct. If there are multiple categories of issues that can make this test fail all should be tested against. Even if it's just to spare Travis a little bit of load...