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`.
Is this necessary? If there is something to be squashed, one would have to interactively rebase anyways and therefore see other occurrences.
I think it's not the same as the other checks (where it would be tedious to get errors shown one at a time).