chore(travis): fail jobs that have a mismatched pubspec.lock

We expect that, for a clean checkout, running "pub get" will not update
the pubspec.lock file.  Any such updates should have been made along
with the corresponding change to pubspec.yaml or must be part of the
commit being tested.  Otherwise, that specific job should fail so the
author can resubmit with the updated pubspec.lock file.