travis: add check to block go binaries from being merged in
travis: add check to block go binaries from being merged in

It happened twice that contributors commited a go binary by accident, and I
missed it during review. It's easy to miss it with GitHub's UI. When
this happens, this is really bad, as we can't afford to carry unneeded
binary baggage in the git history.

Make denying executable files a formal check, so I cannot miss it ever
again.

google/periph