Cater for the corner case where commits are incorporated on a maintenance
branch. We should be able to test these without adding a dev suffix to the tool
versions.
Whenever TRAVIS_TAG is non-empty, TRAVIS_BRANCH is over-ridden to match the
TRAVIS_TAG value. Adjust our tests to reflect this, and add a sanity check that
fails hard if this constraint is ever not met.
See https://github.com/travis-ci/travis-ci/issues/4745
